Why even running a Windows 10 instance in that case? Esp. if it's for backward compat with old crap; run a Win 2k, it will be lighter. And only marginally less maintained, which does not even matter if you isolate like that anyway.
You could request Windows 2000 if you are: Volume License consumer (through downgrade) or Visual Studio subscriber (not for commercial use)
But could you obtain it legally though?
It wasn't hugely advertised, but Win 7 had something called "XP Mode" where it would literally start windows XP in a VM completely in the background, and any program you wanted to start would actually run inside that VM, with windows drawing the application window like a native Win 7 app. It was magical in how well that worked. And yeah, MS just provided a special XP image for this, you didn't need a separate licence for it.