Why no AV1 support yet?

[JuiceWSA]

[Implemented] Add support for av1 coding standard

YouTube has begun rolling out the av1 coding standard on its videos. At least one video downloader passes this through to the final file, making it incompatible with Plex until converted to a compatible format (can be done with VLC). Adding this open, royalty-free video coding format, would retain compatibility going forward