It’s really frustrating when you’d watch a serie, and in all of a sudden an episode is missing. The same problem occurs with collections of movies.
My suggestion is that by means of TheTVDB( https://www.thetvdb.com/ ) missing episodes are showed within the GUI of Plex. There can also be a tab in settings that will show this, so that we can collect (download ) the remaining episodes.
The software for this problem is already available in the program TV Rename.
Link: https://www.tvrename.com/
If the software is not open source, i assume that there is a way through the API of TheTVDB for series and IMDb for movies.
Search first if the request has already been posted before. The goal is to collect the votes for a particular feature in one thread - not to scatter them around.