While this discussion occasionally pops up… the related feature suggestion didn’t catch much attention – you’re welcome to vote/comment in that thread to give it another try (this is by the way the suggestion all those closed/duplicate ones should be pointing at)
I suppose the key reasons why many users don’t care are:
- many/most Plex users are hoarders (in a very positive meaning)

- you can relatively easily replace your actual media file with a dummy one
- simple approach: create a text file, maybe with the path where you moved the file… name it as the original file (incl. the file extension) and replace it → Plex will still recognize the file; after analyzing it will of course find no more technical details (e.g. resolution, bitrate) and it won’t be able to play
- nerd mode: some users even create a tiny placeholder video (less than 1 MB in size)… this one will even play, though the technical details will match the placeholder video (not that of the original file)