>So, is it possible that Google has given up on AMP in Safari on iOS 15 because of the popularity of AMP blocking extensions?

This doesn't make any sense. If AMP is strictly beneficial for google, why would google give up because of a few people with AMP blocking extensions? Surely it's still worth going after the stragglers? You don't see them giving up on ads just because adblock is popular.

I suspect the reason is far more benign: AMP is enabled on a whitelist basis, and they haven't whitelisted ios 15 yet.