There are NO websites - repeat: NOT ONE - where engine-use is effectively policed. RHP used to do it well. Not now, of course. Turn-by-turn, CC-style chess is completely busted, certainly at the higher levels. Even ICCF-server events are engine-dominated.
Playchess is, as Fat Lady says, somewhat different. I used to play there very often. But it's a blitz site - and I couldn't click my mouse fast enough to avoid frustrating losses on time. You also get time lags too. And 'disappearing' opponents.
As I said, serious online chess is busted if you expect to play a human
When I first started playing on playchess.com I used to play 3-0 exclusively (3 minutes each with 0 seconds increment), mainly because Heinzk, who is 20+ years younger than me and who introduced me to playchess.com, used to play that. After a while I realised that this didn't suit my middle aged brain so now I only play 5-0 and this seams to be slow enough for me. As far as I can tell I've never yet met an engine user, but then again I only ever log in as a guest and play ungraded games.
There is no point whatsoever in playing any sort of correspondence chess, whether postal, email or online, at a high level. All the "top" players use computer programs to help them out.