Thanks for replying but that’s not the explanation for this.
You are referring to hardware decoding support for av1. Safari supports software decoding for av1 and should not have a problem on any Apple device running recent versions of Safari.
If your answer was correct then the av1 videos should play in Safari on my MacBook Pro which has an M3 Pro CPU with hardware decoding support, but the problem exists there too.
To further confirm that software decoding of AV1 works fine on older iDevices I checked that the same AV1 videos can be watched via VLC on my iPhone 13 and iPad Pro with an M1 CPU.
I’m certain this is a new issue so either recent changes to Safari or Discourse are behind it. I’ll keep digging and see if it’s been reported elsewhere for anything else.