I looked into this before, and the short answer is that release groups would be allowed to release in AV1, but the market seems to prefer H264 and H265 because of compatibility and release speed. Encoding AV1 to an archival quality takes too long, reduces playback compatibility, and doesn't save that much space.

There also are no scene rules for AV1, only for H265 [1]

[1] https://scenerules.org/html/2020_X265.html

Player compatibility. Netflix can use AV1 and send it to the devices that support it while sending H265 to those that don't. A release group puts out AV1 and a good chunk of users start avoiding their releases because they can't figure out why it doesn't play (or plays poorly).

I'm not in the scene anymore, but for my own personal encoding, at higher quality settings, AV1 (rav1e or SVT; AOM was crazy slow) doesn't significantly beat out x265 for most sources.

FGS makes a huge difference at moderately high bitrates for movies that are very grainy, but many people seem to really not want it for HQ sources (see sibling comments). With FGS off, it's hard to find any sources that benefit at bitrates that you will torrent rather than stream.

h.264 has near-universal device support and almost no playback issues at the expensive of slightly larger file sizes. h.265 and av1 give you 10-bit 4K but playback on even modest laptops can become choppy or produce render artifacts. I tried all three, desperately wanting av1 to win but Jellyfin on a small streaming server just couldn't keep up.

Because pirates are unaffected by the patent situation with H.265.