Blacklisted is the specific terminology for when a website rejects certain file types. I guess in githubs case it would be that webm is not whitelisted since only 2 formats are permitted.

Because it's a real word that is applied in this case.

They're free to use real specific words that aren't actually offensive if they want.

Although in this case I would have thought GitHub uses a whitelist rather than a blacklist, so they should have said "webm isn't whitelisted".

(Notice how that is different to "webm is blacklisted" but your alternative wouldn't have captured that distinction.)