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.
Ragnarok Online. My favorite game ever and it’s all MTX now and will never be like the old days. I wish it was. They made RO classic and shut it down. There’s private servers, but they’re not very populated.