Summerise without changing the meaning:
there are a lot of factors to understand in regard to slavery in the Achaemenid Empire.

First and foremost, there seems to have been little top down legislation for things like social institutions and local economics, including slavery, and royal control of these things became less and less impactful the further you got from the capitals in western Iran and Mesopotamia. There were apparently royally appointed judges that ruled in cases relating to royal laws and decrees, but what exactly that entailed is not well documented. The law codes enforced by the judges were probably an extension of existing Babylonian laws which, as we'll see, definitely did not outlaw slavery. Local law codes and traditions certainly continued to flourish around any royal legislation. Babylonian law codes are hard to work with in this case because they are portrayed as the basis for royal courts, but we also see the the codification of the Torah in Judea, self-governing Greek city-states in Ionia and Caria, and even an Egyptian law code created at Darius I's behest. I bring these four up specifically because there is specific evidence for slavery in all of those places.

There are laws for the proper treatment, sale, and taking of slaves, stories of capturing slaves, and records of slave sales. Some notable examples include a contract for the sale of a slave woman between in the Persepolis fortification tablets:

Razamarma, son of Razamumarga, and Aspumetana, son of Asputatika, sold their slave women Kardara and Patiza to a Babylonian for 2 2/3 minas of silver ...

Judging from the name, these are Persians (or some other Iranian ethnicity) selling a woman to a Babylonian. The records of the Egibi family - an extremely wealthy merchant/banking clan in Babylon - contain many references to household slaves, both in the context of their duties and of their sale. There's also the 4th century Samaria Papyri from the north of modern Israel, which are almost all about the purchase, sale, exchange, and use of slaves. In Egypt, there are similar records in Demotic papyri from Elephantine, with the earliest from the Persian period dating to 517 BCE in the reign of Darius I. There are also letters from the Egyptian satrap Arsames which imply slaves made up the bulk of the laborers on his estates.

We can also look to Greek sources, which should be broken down into two categories. First, definite references to people purchased and held by Greeks or in Greece. The Greek literary record is prolific and often contains references to the names or ethnicity of slaves. Slaves from Syria, Caria, or Phrygia - all in Achaemenid territory- seem to have been fairly common. We also see plenty of names from Lydia and Paphlagonia, though slightly less often. The names from Syria are of particular interest here because that means the slave trade wasn't limited to the region closest to Greek cities, but extended farther into Persian territory. Though, naturally, the Greeks were in more contact with Anatolia. I should also use this as a time to mention examples like Xenopohon's raid on the estate of Asidates in northern Lydia, worked by 200 slaves, and the story of Atys, who comfortably funded a large part of Xerxes' war in Greece because he had large estates operated by many slaves to fall back on.

That last one is recorded by Herodotus is a good opportunity to pivot to the second Greek category here. One of the big pieces of evidence used to sweep aside or deny the existence of Persian slavery is Herodotus' massive overuse of the word. Herodotus uses the Greek doulos all the time when referring to the Persians. According to the first Greek historian, the Persians were coming to turn all of Greece into slaves (douloi). This is about 20% mistranslation and 80% hyperbole. On the subject of of hyperbole: Herodotus had a vested interest in villifying the Persians and portraying them as an existential threat to the Greek way of life. Especially when writing to an Athenian or other more democratic Greek audience that valued their own self determination enslavement was a worst case scenario.

There's also the issue of of mistranslation: doulos does mean slave, but it was also the closes Greek word to the Persian bandaka. That literally means "bondsmen" or "slave" but was also used by the Great Kings (and probably the nobility) to refer to his subjects. Everyone was a slave to the king because the King of Kings owned everything in the world. In practical terms, that just made them subjects like you would see in any kingdom, but in a Greek society that disdained slavery, it sounded much more threatening.

This brings me to the most complicated type of "slavery" in the Achaemenid Empire: the kurtash. This is a thorny word to work through. In the Behistun inscription Kurtash is the Elamite word used as the equivalent of the Persian "maniya" which means "household servants" both slaves and free. However, in the much more numerous Persepolis tablets, kurtash refers to workers conscripted by the state, forced to leave their homes, and made to work on royal estates or building projects. On one hand, that sounds a lot like slavery. On the other hand, these people could not be employed in private projects (excepting the royal family because they were the state), could not be sold as property, and at least theoretically had a term of service and returned home. So are these slaves? I guess it depends on your definition. It was forced labor, but they were no more property than they were just by virtue of living in the empire. In that regard, it's a lot more like a civic obligation in addition to their taxes/tribute.

Finally, there's your example, a reference to enslavement or deportation after the siege of Miletus. Deportation brings up one last key category. Prisoners of war, especially but not only after revolts, would be sent to far away parts of the empire. At least in the case of the Greeks - because they kept track of this and had their records have survived - many of them wound up in Bactria and Karmana. These deportees would first have been used as forced labor. As usual that can be called slavery, but they served a very similar role the kurtash in practice and would ultimately be settled free of forced labor other than what was expected of them as royal subjects.

Now, up to this point, someone who knows a lot about the timeline of things and people in the Persian empire will notice that I still haven't technically answered your question. Almost everything above dates to after Darius I the Great came to power because that's when we see most of the records. There are a few Babylonian documents dated to the reigns of Cyrus and Cambyses, but not much on this topic. The one descent example we have is from another story of deported Greeks resisting Persian rule in Ionia. Around 540 BCE, one of Cyrus' generals (Mazares) besieged and sacked Prienne. The population was "enslaved" and removed according to Herodotus, but there's some debate over whether that means deported, and Herodotus is exaggerating, or if they really were taken and sold. The latter would be very out of character for Medes and Persians in the 6th century, but not impossible, especially if they were dealing with Greeks and sold them to other Greeks.

Ultimately, we don't have any information about Cyrus' policy toward slavery itself, but we do have plenty of evidence for what his policies were in general. Cyrus generally seems to have adopted the local status quo whenever possible. In Media, he married into the royal family and let Astyages live out the remainder of his life. Lydia he tried to leave local governors in charge until they revolted. In the Ionian Greek cities, he let each city govern its territory as a polis under a pro-Persian tyrant. In Babylon he adopted all of the religious and cultural traditions of Babylonian kingship. In Judea, his proclamations were issued in the name of the Jewish God. Cyrus seems to have left every system as untouched as possible.


As for any religious basis, that is very unlikely. First of all, we don't know much of anything about Cyrus' religion. There's actually nothing to indicate he was a Zoroastrian other than Zoroastrian tradition and its historic dominance in the region. The only religious statements associated with Cyrus are about Babylonian gods and horses sacrificed at his tomb. Horse sacrifices are more closely associated with Mithra than any other Iranian divinity. Even if he did acknowledge the supremacy of Ahura Mazda and the revelation of Zoroaster (to give a very loose definition of Zoroastrianism), the Gathas and other early Avestan texts are mum about slavery. It just isn't brought up. Much later, Sassanid law forbade the enslavement of fellow Zoroastrians, but was more than happy to permit the enslavement of others. That might be part of the root of your original misconception.

The greater part though is the reputation of the Cyrus Cylinder as "the first universal declaration of human rights." This is utter nonsense issued as propaganda by Muhammad Reza Shah Pahlavi, the last Shah or Iran, in the 1970s. Since then it has been repeated high and low and been the basis for far too many fake translations online. This compounds by pointing to some similar language between the Cylinder and the Biblical book of Ezra. All of this is premised on a single passage in the much longer cylinder about allowing people and idols held captive in Babylon to go home. This does not mean slaves. At best, it means people like the Jews who were deported to Babylonia. More likely than that, according to most modern scholars, it refers to the idols and priests taken into the city by the Babylonian king Nabonidus immediately before Cyrus' invasion. Here's the section:

30. From [Shuanna] I sent back to their places to the city of Ashur and Susa,
31. Akkad, the land of Eshnunna, the city of Zamban, the city of Meturnu, Der, as far as the border of the land of Guti - the sanctuaries across the river Tigris - whose shrines had earlier become dilapidated,
32. the gods who lived therein, and made permanent sanctuaries for them. I collected together all of their people and returned them to their settlements,"
33. and the gods of the land of Sumer and Akkad which Nabonidus – to the fury of the lord of the gods – had brought into Shuanna, at the command of Marduk, the great lord,
34. I returned them unharmed to their cells, in the sanctuaries that make them happy. May all the gods that I returned to their sanctuaries,
35. every day before Bel and Nabu, ask for a long life for me, and mention my good deeds, and say to Marduk, my lord, this: “Cyrus, the king who fears you, and Cambyses his son,
36. may they be the provisioners of our shrines until distant (?) days, and the population of Babylon call blessings on my kingship. I have enabled all the lands to live in peace.