Pilot here - it's legal to fly at 12,500 in an unpressurized aircraft indefinitely. See https://www.ecfr.gov/current/title-14/chapter-I/subchapter-F.... Cabin pressurization is set to a lower altitude, but that's not for a legal requirement. It's just nicer to be at higher pressurization.

And, I mean, think about it - there are _towns_ at 9k+ feet.