Based off of this Steam post, it looks like they still do:
There's a new option on the Support page that lets you delete game from account. However it doesn't remove the achievements and deleted games still counts towards % game completion. This should be fixed.
I also found a rather detailed Steam Guide about the Average Game Completion Rules and it states:
If you earn achievements for a game that is not in your library for whatever reason, Steam still remembers them and they WILL still count. This includes achievements for games you played during a free weekend but never eventually bought!
It goes on and says:
In the same vein of Rule 3, it appears that removing a game by having it permanently deactivated from your account via Steam Support also does not remove the game's percentage from your AGCR. In other words, even deactivated games still count.
Bottom line - yes they will still factor into the average, which in my opinion makes sense still since at one point, you did play the game.