I wonder about this a lot. If ever such a distinction did happen, it would almost certainly be because of certain authorities seeking to outlaw non-KYC bitcoin.

Of course this would not bode well for fungibility, but I wonder would such a scenario even be possible with a borderless, decentralized protocol like bitcoin?

Plus it's fairly trivial to obfuscate ownership. Perhaps in the future it will not be so easy and then maybe non-KYC bitcoin could sell for a premium. But in that case I can also see non-KYC bitcoin becoming less valuable if using it risks breaking the law.