Here is the current status of porting omv-extras plugins to OMV 6.x. If the plugin isn't on the list, it won't be ported.
Whoah ... JellyFin's just gone now?
(later edit) this may be a dumb question cause I'm new. But I could have sworn there was a openmediavault-jellyfin plugin at one point, only now I can't find it anymore.