I know this has already been mentioned over numerous topics (and I won’t even bother linking them this time), but I think that for the sake of systematicity it’s best to make one coherent topic focused on the issue. For some reason one-handed mode is missing in Android 12 on FP4. The toggle should be in Settings → System → Gestures, but it’s simply not there. And no, this is definitely not a Pixel-specific feature:
This was one of the most anticipated features of Android 12 that FP4 users actually wanted. It’s really disappointing that it’s missing.