In my case, I'm running FreeNAS/TrueNAS for storage and a Plex jail. I'd imagine there are a number of people in the same boat considering the popularity of TrueNAS.
TrueNAS Scale is Debian-based. It's easy to migrate from Core to Scale [0]. You do lose your jails but there's both an official Plex chart for the underlying k3s, and a community-supported one in TrueCharts [1].
TrueNAS is a pretty small niche, no? Not only that, but the usual way of using NAS involves doing compute tasks on a separate server.
Besides, the FreeBSD based TrueNAS Core will almost certainly be deprecated and replaced with Linux based TrueNAS Scale within the nest couple of years. Further strengthening the idea that this is a pointless platform to support.
There's a Jellyfin chart on TrueCharts as well.
[0] https://www.truenas.com/docs/scale/gettingstarted/migratingf...
[1] https://truecharts.org/