Anyone who interested in this, please take 'https://cammytang.github.io/index.html' as fallbackURL to test. Thanks.

Good news. Works on MacBook Pro Version 12.0 Beta 4, iPadOS 15 Developer Beta 4. ( Beta 3 doesn't work)

Hi @greg, shareplay is re-enabled in macOS 12.1 just as you guess. But we still can't create an experience on our website by using safari. Because APIs of navigator.mediaSession on safari are still not enough. There is no navigator.mediaSession.coordinator api. See https://developer.apple.com/videos/play/wwdc2021/10189/. Do you guys have any progress?