One of the games in which I'm involved stayed with PF1. (The other game played PF2 through the playtest and for several months after the release, then moved on to 5e.)
I don't really see any major pieces of PF2 as tempting to bring back to PF1. Several months back I would have said the "three action" system, but now I don't see it as that big of a win. It's a little simpler conceptually, and it had neat ramifications for spell casting, but you wouldn't get that with porting it to PF1. It would help with multiple attacks. So, I guess I'll flip my opinion and say "maybe" we'd consider bringing the three action system back to PF1.