When trying to play AV1 files using plex tvOS client, transcoding is initiated to h264.
Create Profile folder and copy the tvOS.xml file to the folder and restart your Plex server.
Location of Plex data:
/var/lib/plexmediaserver/Library/Application Support/Plex Media Server/
mkdir "/var/lib/plexmediaserver/Library/Application Support/Plex Media Server/Profiles"
nano "/var/lib/plexmediaserver/Library/Application Support/Plex Media Server/Profiles/tvOS.xml"
Copy contents from tvOS.xml to new file, save and reboot Plex server.
Test Plex client on Apple TV.
For help finding Plex data location on other Operating Systems:
https://support.plex.tv/articles/202915258-where-is-the-plex-media-server-data-directory-located/
Tested with smooth playback on latest Apple TV 4k and last gen model. 1st Gen model stutters on 1080p av1 and 4k content.