To which playlist is the file already added

There’s an existing feature suggestion asking about an option to indicate in which playlist an item is included (incl. mockup in the last post).
https://forums.plex.tv/t/indicate-if-an-item-is-already-on-a-playlist/361431

There’s also an existing suggestion to manage the playlist association through tags (as per your suggestion)… there’s even 2 “flavors” of this

If that’s what you’re looking for, I suggest you vote/comment in those threads and we close this thread in order to avoid distracting/dispersing the attention (=votes)