How exactly do you imagine running PPC OS on a PC?
I do recall that you could run Mac OS Classic on an Atari ST (both 68000) - you just needed 'genuine' ROM chips from a Mac (I'm sure there were ways and means to obtain quote-genuine-unquote ROMs) - and if you could do it on an Atari ST then someone would have come up with a way to do it on Amiga with added bouncing beachballs and funky sound samples (it's the law!)
Also, back in the 90s, Windows NT briefly supported PPC (and other) alternative chips - if this had succeeded and the market was flooded with cheap PPC boxes then I'm sure we'd have seen PPC Hackintoshes. However, as it was, Macs weren't particularly expensive c.f. other low-volume PPC hardware.
However, Windows on non-intel failed (either because Intel and MS deliberately strangled it at birth, or simply because nobody ever chose Windows because they liked the OS - its selling points have always been "cheap hardware" and "runs my 20 year old software natively").
Only one clone that I was aware of and Steve Jobs killed that very quickly when he returned to Apple.
No, there were several, and they were rather successful. Trouble is, at the time, Apple were still trying to make a significant chunk of their money from the high-end desktop workstation market, which the clones threatened in a way that the current "Hackintosh" doesn't.
List of historical Mac clones:
https://lowendmac.com/the-macintosh-clones/
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I don't even think that Apple would have to build any particular safeguards agains running macOS on foreign hardware.
Most likely, Mac OS 11 will just require, from the start, that ARM machines have the T2 chip (or equivalent) which will probably rule out ARM Hackintoshes with the same requirement for Intel machines appearing as soon as Apple feels they can stop support for iMacs sold in 2020.