Full disclosure: I work for Tailscale.

You’re looking for custom OIDC providers.

https://tailscale.com/blog/custom-oidc/

What other option do we have? Either stop using it or attempt to get Plex to respond and change their ways.

Right - it’s controlled server side by Plex and so far as I can tell there’s nothing you can do. Your friends would have to go in and turn it off in their accounts. It’s disgusting.