• Longbridge - Win10 modded to look like Longhorn
• Classic 7 - Win10 modded to look like Win7
• Projek01 - Win10 modded to look like XP
• eXPerience - Win10 LTSC 1809 modded to look like XP

Note: I haven't tested any of them, and I don't plan to. Nevertheless, I want to catalog them here because they're interesting.

What I don't like in particular is that, as pointed out by The Register, even winver lies and says the system is Win7/XP, and you can't install any updates from Microsoft.