Lee Carré (2021-09-25 21:08:12+00:00): “Given that StreetComplete makes it very easy and comfortable to update details, while those changes require a lot of time and effort in Vespucci and other editors, I lean towards accepting that the source might not be 100% accurate in all the cases.”

Convenience doesn't trump accuracy.

Most convenient is to not edit areas for which source=* can't be set accurately (or, in case of SC, which you aren't able to survey). SC gives a warning, when more than some metres from the element, for this very reason.

However, you may simply have to wait a while. I recall reading on GitHub talk of SC setting source conditionally (e.g. based on distance between user & element).

You being willing to accept inaccurate data doesn't mean that it's acceptable for others. One of the differences is that YOU KNOW that it's inaccurate; others have no way of knowing that (given misleading value of source=*).

OSM has a core principle of verifiability. Setting source=survey indicates that you were there (at the time) when you've admitted that you're not.

“I'm not sure if the strict differentiation between "local knowledge" and "survey" is very helpful.”
&
“My point is: what purpose does the differentiation between "local knowledge" and "survey" serve, when it is so hard to define?”

Besides these two sentences being mutually-contradictory; you being unclear on the distinction, or having difficulty defining them, doesn't make that an absolute. That's a logical fallacy; argument from ignorance.

“At what point does a survey become local knowledge? When I have walked a street earlier today and update details about it at home, is it still a survey or already local knowledge? And if I go back to that street and update it while looking at it, is it a survey even though I already knew all the details I was updating?”

That's a much more (intellectually-)honest approach (asking open, non-loaded questions) than making assumptions or holding presuppositions.

I don't (personally) decide such things. I follow the consensus-established conventions described in the wiki. For this matter, see http://wiki.osm.org/wiki/Key:source

I have enough experience (generally, rather than of OSM) to know that sometimes there are good but non-obvious reasons behind some rules.

I would invite you to ask such questions in a more prominent place. Such as the mailing lists, or on help.osm.org .

I can only give my own opinion on & interpretation of, the matter.

To me, it's an indication of (un)certainty. Likely stemming from good practise with data-handling, and scientific rigour.

Misleading data will result in mistakes. Sometimes then leading to further mistakes.

Being diligent takes effort, which takes time.
If you're unwilling, then refrain. Leave it for someone else. Drop a note, instead (to draw attention, for locals to address).

I've seen, several times, incorrect assumptions being applied by well-intentioned people.

My point, in general, is that there's then a risk of unknown things going wrong.

But, some specific & practical issues:
▪︎things may have changed since you were last there
▪︎being familiar with an area is different to closely examining it for the purposes of mapping; I perceive & traverse my local area very differently while out surveying than when merely passing through
▪︎one differentiator would seem to be the extent or degree to which the mapper is relying on his own memory (which is generally lousy). Before mobile apps, surveying involved recording in other ways (textual notes, photos, video, audio of the mapper describing what he saw); thus when back home and inputting the data, he's using recorded data rather than his own memory
▪︎a somewhat meta-factor is to know if the timestamp is (at least close to) when the observation was conducted (though there are keys for setting that in an explicit tag)
▪︎in the case of disputes, a survey will always trump mere familiarity with the area; I've encountered things which were wildly wrong when stood there, compared to OSM's data.
▪︎imagine an armchair-mapper finding data which doesn't match imagery; well, if the data is from survey (or a more recent / accurate source), then the latter trumps (I've encountered this one, too)

There are probably more reasons. Though, defying convention will likely need a compelling case. There was obviously a reason why the convention was established.

Consider it good practise. When using Vespucci, I only set source=survey when I'm actually there. If I'm elsewhere (back at base) then I'll set something else.

You may have a point. It may now be something which made more sense in the past. But, don't assume that. Ask the questions in a place that'll be seen by more experienced and/or knowledgable people.

It may be that in the past, before mobile apps, it was acceptable to upload data from a survey done several days prior. That being a limitation of what was possible at the time.

However, now, maybe the definitions can be more strict. Instead of source=survey meaning that you WERE there, now it soulful mean that you ARE there (at least when inputting the data (or answering quests in SC), to allow for uploads delayed by a few hours when the user has a suitable connection again (I say that as one who has often done a day of surveying with SC and uploaded everything via 802.11 when back at base)).

In one of the scenarios you're hinting at, maybe it could be solved by setting a tag stating when the data was collected / input, as distinct from when the changeset was opened / closed.

Personally, the temporal threshold between survey & local knowledge would be 24 hours (though I could be persuaded to reduce that).