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.
Witcher 3. I don't enjoy "dark" "gritty" stuff the way that I used to. I'm way more effected by some of the more explicit themes than I used to be. When I played it it was my only solace in a rough period of my life. I'd rather protect my fond memories than play it again, as I doubt it will mean as much to me now as then.