This could be bro science, but I'm fairly positive that repeated blows to your bones strengthens them over time, as long as you're not overdoing it.
My anecdotal experience is sort of murky, because I started doing martial arts when I was still going through puberty, but my knuckles are super tough. Of course it could be the result of learning better form and gradually getting used to the pain, or maybe even killing off some of the nerve endings that should warn me that I've experienced an injury. It's hard to say.
I've definitely known muay thai guys that ostensibly sought to strengthen their shins over time by kicking harder and harder things, but again, possibly just bro science.