App verification is only enforced on ROMs that honor it. You can choose to patch it out of the OS as the maintainer. Which will bar you from being a google certified OS.

The people behind GOS have gone on record, stating that they don't care about being google certified, and GOS will not enforce dev verification. So this changes nothing for GOS.

To stop GOS, google would need to change their bootloader policy. I haven't heard of any indicators for this though.

In summary, you're fine to use it on new and used Pixels. Given current information.

Reference: https://discuss.grapheneos.org/d/26337-android-developer-verification-are-we-screwed

Yes.

Eventually there will be a non pixel phone that works with grapheneos. This has already been announced.

It seems to be a concern to the people behind GOS, enough that they are looking for an OEM to make phones to their specs, but I haven't heard anything to the effect of "Google will stop all support or brick devices that aren't verified." Maybe existing GOS Pixels will be fine, maybe they're sandbagging, I don't know. It'll definitely suck for me with a Pixel 9 Pro XL that I expected to keep into the 2030s if that happens.

Not 100% sure but I believe this is to prevent side-loading on stock android. That is, a normal android can only install from the play store. Graphene and Calyx should be unaffected.

Someone correct me if I'm wrong.