So I have noticed there are some quests that are still available like they can be completed for example "complete this week's Nightfall: The Ordeal on adept difficulty" from Zavala. I think it was a season 10 quest.
Anyway it doesn't make sense to keep the coding for those in game because its expired so it no longer functions. Not game crippling or anything, just an observation, thought I'd mention it.
-
If you are unable to complete a quest from a previous season, it can be safely abandoned.