The first game that comes to mind for me is Civilizations 4. I've probably spent hundreds of hours playing but after getting used to 5 and 6 I have a really hard time going back. Going back and forth between 5 and 6 I need to rethink some strategies but with 4 I feel like I need to rethink everything. I don't know if it's because of the tech tree layout in combination with the civics system, things like unit stacking, or maybe just a bunch of little things but it takes me a while to readjust.
I loved a lot of early Blizzard stuff and I have no desire to play any of them ever again. Warcraft 2, Starcraft, Diablo were all huge parts of my life and I have done nothing with any of those series since those games, let alone go back to those.
I was also a so into Smash Brothers that I was in the competitive scene and won some locals, but I haven't really felt the call of that series since Melee. Loved my time with the game and the scene though.