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.
Most games I played from the PS1/N64 era. It was a time period of figuring things out and that makes it rough to go back to, but some of those experiences were magic. There’s still the rare game that I can enjoy to this day (Metal Gear Solid 1, Mario 64), but games like Syphon Filter are best left to my memories.