[Google Pixel 2 Guides, News, & Discussion] JangoSteve:
I just read this entire thread beginning to end (not skimmed, not searching through it, but actually read it), all 779 posts. At least a quarter of it is people asking the same questions over and over, since obviously no one else is crazy enough to read through a 779-post thread to have their questions answered.

I'm no expert, but in case it helps others and also to prevent myself from having to read this over again when I inevitably forget everything, I'll summarize what I've read here.

1) Verizon locked down their Pixel 2 so that the bootloader could not be unlocked. This is separate from being carrier-unlocked, which is what most people are referring to when they sell an "unlocked Pixel 2", so if you're buying a Pixel 2 with an unlockable bootloader, make sure that's specified.

2) The non-Verizon Pixel 2s that people receive from RMA replacements from Google also seem to have bootloaders that can't be unlocked. Some people have gone through 5 RMAs or more and countless calls with Google support to get a Google replacement with an unlockable bootloader, some many gave up, a couple eventually got refunds after many failed replacements.

3) The method used to prevent the booloader from unlocking seems to be the same, or highly similar between the Verizon phones and the Google RMA phones, which is why there are plenty of non-Verizon Pixel 2 people in this thread.

4) There were a couple Pixel 2 "walleye" Android 8.1.0 patch versions (.011 and .013) from December and January which had a hole in the unlock prevention code that allowed the booloader to be unlocked by running the "critical lock" command through ADB. This was "fixed" in the .016 release of 8.1.0 and beyond, never to return.

My understanding is that this code is intended to lock the bootloader, not unlock it, which is probably why it was missed in the code that is supposed to block this ability. My guess is that in ADB, the command was probably written as a toggle instead of straight up locking the bootloader (i.e. "if bootloader is unlocked, lock it, otherwise, unlock it"). This is just me attempting to fill in the blanks though, as nothing in this thread really goes into this level of detail.

5) If you have ANY version of 8.1.0 patch .016 or beyond, even if it's Android 9 or 10, you're out of luck, as there is still to this day NO way of unlocking the bootloader (unless you have a non-Verizon and non-locked-Google version of the Pixel 2, since even some of the Google versions come locked now too).

6) With an unlocked bootloader, you can load whatever versions of the factory images or other ROMs you want, so you can move back and forth in Android versions. With a locked bootloader though, you can only move forward in Android versions, never backward. So if you got a Verizon Pixel 2 on 8.0.0, you can update it forward to the vulnerable versions of 8.1.0 in order to then unlock the bootloader, but you can't go from a later version of 8.1.0 (or Android 9 or 10) backward to the older vulnerable versions of 8.1.0 to unlock the bootloader.

7) If you're going to update your new Verizon Pixel 2's Android version as in (6) you probably want to make sure that you do it right out of the box, but that you first turn off wifi, don't insert the SIM card or activate the phone, because as soon as you do, the phone will try to take over the air (OTA) updates that zoom you right past the vulnerable versions that allow unlocking the bootloader. And, as stated in (6), once this happens and you go past those versions, you can never go back, so it's game over. So, the recommended way to update forward to the vulnerable versions is to use ADB sideloading/fastboot with the factory OTA or full images, respectively.

Likewise, if your Verizon Pixel 2 comes with a later version out of the box, then you never even had a chance, and there's no way to unlock the bootloader no matter what you do. The firmware version of the phones are written on the box, so you can check before you buy, assuming the phone wasn't returned or previously opened and powered on by a sales associate.

8) If you got a Verizon Pixel 2 that allows you to toggle the "Allow OEM Unlock" setting in the Developer Options in Settings and isn't grayed out, that doesn't necessarily mean anything. A lot of the phones have this option available out of the box, but still won't let you unlock the bootloader even when you toggle the setting to enable it. The consensus is that there is some other check that Verizon has the Android firmware doing that overrides this setting. When you try to unlock the bootloader, the command will fail and then the setting will become grayed out. If you do a factory reset, it'll go back to being able to be toggled, but then actually trying it will gray it out again.

9) Similar to the "critical lock" command described in the OP to unlock the Pixel 2 on the vulnerable Android builds, there is also a "critical unlock" command. The Google documentation says that this command is ONLY to be used on the Pixel 2 XL, but no one knows why.

A couple people have tried it on the Pixel 2 with varying success, and it's unclear if the successful ones would have been successful anyway with the "critical lock" command. Futhermore, no one knows why it shouldn't be used on other phones. It could be due to the specific hardware of the phone, or maybe it's just not well tested. The main advice is to heed the Google documentation's warning and not to use it on the Pixel 2. And yes, the Pixel 2 is a fundamentally different phone than the Pixel 2 XL, since the Pixel 2 is made by HTC and the Pixel 2 XL is LG.

10) There was quite a bit of effort by one person in this thread in particular to try to figure out the exact mechanism that is overriding the OEM unlock ability on the Verizon and RMA Pixel 2's, in an attempt to find an alternate way to unlock the bootloaders on the phones that either came or were updated past the vulnerable Android versions before the bootloader was unlocked. However, despite much progress and a valiant effort, nothing came of it and they eventually moved on from the Pixel 2 to other phones. Unfortunately there was no slam-dunk replacement for the Pixel 2 in terms of features, unlockable bootloader, performance, updates, etc.

11) Once you do unlock the bootloader, no OTA update should ever re-lock your bootloader. While it's technically possible, doing so requires wiping all user data from the phone, and carriers would cause a PR ****storm if they pushed out an OTA that wiped people's phones without their consent.

12) If you do manage to unlock your bootloader, DON'T re-lock it. If you do, you're removing your ability to flash back to known working images if something should go wrong, since locked bootloaders only allow moving forward. Even worse, you could end up bricking your phone if for instance you re-lock your bootloader with a modified boot image or root on your phone, since the verification checks required by a locked bootloader to boot your phone may no longer pass.

13) There are sometimes weird errors running ADB/fastboot commands that can be caused by certain USB ports and cables. Some people have reported that USB-C to USB-C cables don't work for them, but switching to a USB-A to USB-C cable made the errors go away. Some people report that USB 2.0 and USB 3.2 ports worked, but USB 3.0 ports did not, while others say their USB 3.0 ports worked fine for them. A couple people couldn't get commands to work on one computer, but got them to work on another.

The point here is, if you're following the guide closely, and your platform-tools with ADB and fastboot are all up to date, but you're still getting weird errors despite doing everything correctly, try different USB ports on your computer, different USB cables, or even switching to another computer.

14) Having an unlocked bootloader shouldn't affect your ability to take OTA updates once you're unlocked (I'm not an expert, so I can't take responsibility if it does affect OTA updates, I'm just echoing what I've read here and using my own understanding). However, if you've actually done anything with having an unlocked bootloader, like using it to patch your Android boot image to allow rooting, or modifying system files or anything, then you can no longer take OTA updates straight up. You now must use Magisk or ADB to side-load updates supplied by Google on their image or OTA download pages.

15) Other info about rooting, taking updates when you're rooted, using Magisk or TWRP, etc. are outside the scope of this thread. Once your bootloader is unlocked, you can basically treat your Pixel 2 as any other bootloader-unlocked Pixel 2, and so you can follow the generic Pixel 2-specific guides on rooting, updating Android while rooted, modifying your phone's software, etc., with the exception that you can't follow their instructions on unlocking your bootloader in the first place, and you also should NOT re-lock your bootloader, since unlike the actual bootloader-unlockable Pixel 2's, you can't just re-unlock them later if you've updated beyond the vulnerable versions of Android on the Verizon/RMA Pixel 2's.

I haven't said anything here that hasn't already been said in this thread several times, except some of my filling in the blanks in (4). But hopefully, this helps anyone coming across this thread and not able to read through the entire thing as I just did, or anyone that has read through it but can't remember it all.

If I've missed anything or gotten anything wrong, please feel free to add or correct me. I went from not understanding much about unlocking bootloaders to feeling like I have a decent understanding, so thanks to everyone who contributed to this thread.