I wish that when you clicked on a show poster in the TV Section you would jump directly to the next unwatched episode in order of oldest to newest. This would be very helpful for when you have shows that have multiple seasons, skips having to go through so much clicking and browsing.
I know you can filter and sort but that doesn’t really resolve the overall issue.
You can change the time frame how long abandoned tv shows are showing up in OnDeck.
Settings - Server - Library - (Show Advanced) - ‘Weeks to consider for On Deck’
Besides, if you pick a tv show from the complete list of tv shows, and simply click on Play on the left side, then playback starts with the next unwatched episode. There is no need to find and dive into the right season.
(I assume you are using the Smart TV Plex app. If not, please specify which Plex client you are actually using.)