@nello, if you have the old version in your “pkg cache”, you can install it from there, and then “lock” the package jellyfin to prevent it from updating.
Once TrueNAS Core releases 13.3 (which will be based on FreeBSD 13.3), we can then upgrade our jails to FreeBSD 13.3 and use the latest Jellyfin version.
If you don’t have a copy in your pkg cache, you can fetch the binary package from this URL:
https://pkg.freebsd.org/FreeBSD:13:amd64/quarterly/All/jellyfin-10.8.13_1.pkg
To fetch it while inside your jail:
fetch https://pkg.freebsd.org/FreeBSD:13:amd64/quarterly/All/jellyfin-10.8.13_1.pkg
If the quarterly repo gets updated, and thus this older version of Jellyfin is unavailable, you can still grab a copy from archive.org by supplying the same URL above and downloading the file from their snapshot.