The settings under Agents are not relevant for newly created music libraries.
All relevant settings are now available when you edit the library and go to the ‘Advanced’ tab.
There, activate ‘Prefer local metadata’ if you want Plex to honor your embedded meta tags and set the rest according to your liking.
All development is now happening in the new “music code”, which uses musicbrainz.org as its main data source (with allmusic.com and last.fm embellishments).
If you want your library to use this code, you need to edit your music library and set both Scanner and Agent to ‘Plex Music’.
But careful: Plex expects a folder structure of Music > Albumartist > Albumtitle > Tracks
If your file organisation doesn’t conform to this, mismatches and general weirdness will ensue. i.e. it is now even more strict about file organisation than it used to.