> They could do what Windows has done, and build OS code that checks if the running application is a known-legacy game, and lie to the game about various capabilities so that the game runs well and looks good.

Or, even simpler (and AFAIK modern Windows does that too): if the running application doesn't say in its application manifest "I'm a modern application which understands the new things from operating system versions A, B, C and features X, Y, Z", you know it's a legacy application and you can activate the relevant compatibility shims.