Is ECC RAM worth it for a Plex server?

Probably not. Plex is a media streamer, not a database server, so it doesn’t care about random bit flips. You’ll never notice a flipped bit in a movie frame, and system crashes from memory errors are extremely rare. On older hardware, ECC RAM often costs more and may be slower, and you’re better off spending that money on more storage or a newer, more efficient machine.