If the VPN does not support port forwarding is it still possible to use for Linux torrents?
Yes with caveats. Torrent swarms need at least 1 connectable (port forwarded) peer for the swarm to exchange data. If all the peers are firewalled (not port forwarded) then all the swarm peers can see each other but cannot exchange torrent data so there will be no uploading/downloading in that swarm.
Generally speaking you won’t notice much difference in large torrent swarms since those swarms usually have some/many connectable peers. But in smaller torrent swarms you may have trouble since your odds are worse that you’ll find connectable peers in those swarms.
PS - Yes you are still seeding/uploading while firewalled (not port forwarded) just not very effectively. While firewalled your best connections will be with connectable peers in the torrent swarms. Not much to think about with public torrents but it’ll kill your ratio at private trackers for sure.