I mainly watch episodes from the Recently Added Television section or On Deck. However, I have a lot of TV episodes added in a short period of time and the RAT section is quickly filled. One of the biggest problems is that even those episodes I've watched are in there and I've soon ended up in a "can't see the wood for the trees" type situation.
My primary Plex app for the the last 18 months has been Plex Home Theater and more recently Open Plex Home Theater. I love that in there the option exists to no longer show episodes in the Recently Added List once they've been watched. This really streamlines the experience and makes it easier for me to keep up to date with the latest shows, particularly new ones which I've set up in advance of airing and so have no history to make those episodes automatically show up On Deck.
It would be great if you'd consider adding this option to the Apple TV Plex app. Even if it is disabled by default I would love this functionality.