Leaving bugs unfixed for years.
The 8 Crossbowman card e.g. was bugged in both campaigns and skirmish/mp. While it has been fixed for the latter, it still remains bugged in the campaigns.
Or after ranked treaty has been readded as lobbies, you loose Elo despite winning the game.
Edit: Or in general, leaving AoE 3 out. On the promo material for 2022’s Red Bull, they even featured a AoE 3 screenshot despite the game not appearing there.