You run them in a virtual machine running an older version of the OS, the same way you do with legacy Windows apps that won't work on modern versions of Windows.

Which is overwhelmingly terrible compared with native. It seems there is a never ending set of copy/paste, window resize with multiple monitors, etc problems.

And it only works when your application doesn't have HW requirements that can no longer be met. Or the company in question just doesn't provide drivers anymore.

Frankly, the driver thing is somewhat understandable, but as you point out its easy to bolt software compatibility layers on, why the OS vendor can't manage to make it work well/transparently is a mystery.