is there proof somewhere that kankri was raised by a highblood other than it being said that that's how beforus works? like does he ever talk about it?

Kankri doesn’t talk about the highbloods he was raised by but since he’s a mutant we can generally assume that’s what happened. A lusus wouldn’t choose a mutant to take care of unless it was specifically bred for it like Karkat’s lusus was.