Theres another script that deletes watched videos from the specific youtube plex library every night.
It works great as long as you remember to remove the videos from your watched later playlist before watching them on plex. (Have yet to find out a simple way to automate playlist removal)
<https://github.com/mps-youtube/mps-youtube>
It was (well, still is) a terminal-based YouTube client which provided search, locally-managed playlists, choice of playback mode (video or audio-only), downloads, and a number of other features. YouTube quashed it with an API-key limitations such that it is now effectively unusable.
But for a brief moment, YouTube content was accessible from a terminal window or console, without all the godawful dark patterns of the site itself. That was tremendously useful.
These days I'll snag videos by URL via youtube-dl/ytdl, and/or mpv (which invokes those transparently). Not quite as magickal, but still pretty good.
Hrm... Apparently there's a successor project, yewtube:
<https://github.com/mps-youtube/mps-youtube/issues/1191>
<https://iamtalhaasghar.github.io/yewtube/#installation>
I'm going to have to check that out...