Plex killed their official plugin repository, but plugins are, technically, still supported. There just isn't much life left in that ecosystem after Plex strangled it.

Ironically, it's probably Jellyfin's thriving plugin-ecosystem that's holding back its clients - since anything with a native UI can't really be used with any plugin that extends the UI feature set and vice versa.

Oh, and all "workarounds" that I know of for "offline" Plex involve essentially disabling user auth for certain IPs - which is insane. Plex simply doesn't support local auth, it's not an offline-capable solution. That (and some other restrictions) is why I'm still running and maintaining Jellyfin as a fallback.