There are plugins for SSO.
There are 3rd party plugins for OIDC and I think LDAP is even first party.
The issue comes when intercepting the signin-progress with 1st party clients. Jellyfin (to my knowledge) doesnt support redirects/callbacks like a homeassistant companion app does.
And how many media servers are there? The 2 other major offerings (Plex and Emby) don't support OIDC either.
Plex does it's own sauce and Emby doesnt support it. Authentik has a guide to implement it via LDAP.
And Jellyfin has a tech-debt history being forked from emby. Stark contrast to newly developed projects which were started when SSO and OIDC wasbstarting to become popular.

This sounds lile you are very knowledgable about it.
Why not propose a dev-draft or propose a feature on their feature voting website?