I don't think it legitimizes censorship. Rather, it makes censorship obvious and unmistakable. A generic status code (like 403) is indistinguishable from a technical error and obfuscates the existence of censorship.
It seems undeniable that this "feature" codifies censorship, making it more "acceptable".
"Oh, well they used the right code anyway"
back to approved activities
So, in the case at hand, GitHub is using 451 to communicate that a government censor has required that they take some content down, not that the company itself has decided to remove the data of its own volition.