I use custom Collection Tags to create smart playlist to play shows with crossovers in the correct order, for example the Arrowverse DC TV shows.
But I also have no intention of keeping old episode of these shows in my collection, so I also have “delete watched episodes after 1 week” for these shows too.
The problem is, if all episode get removed before the next episode is in my library, the Collection Tag gets removed from that shows metadata, so it doesn’t get included in the auto playlist.
How is the collection supposed to know, to which tv show it has to apply itself?
Here is a different way without collection tags:
So as soon as there are no more episodes within a shows folder Plex Server purges all metadata from the whole show? And then when a new episode is added it starts from scratch matching the show? There’s no way for the tag to persist as part of the shows info, of the tag is custom?
I didn’t realise that was the case. That’s a shame.
In that case I’ll add a vote to the “longer delete watched period” feature request further down the forum.
I cannot comment on the part answered by Otto about watching crossover episodes of several series in order. As for your specific request… you can now directly create “smart collections” based on filter criteria. This requires PMS 1.22.3 and Plex Web 4.56.3 (currently only available in the hosted version of Plex Web or Plex for Mac/Windows 1.31.0). Such a smart collection will stay in your library even if it has temporarily no items.
Not 100% if this will cover the original request 
