This seems like a poor argument. I don't like much either having the obligation to give GitHub my phone number, but it's not the same thing as a social security number, now is it ? Would you argue otherwise ?
Not US but phone number is arguably worse: You can't legally get one without tying it to govt ID anymore and tends to be tied to your current physical location.
And is commonly used for authentication codes, and like Social Security Number, it is PII that should be default-deny.
Github seems to have no legit need for a user's phone number. Since there's not even a way to tell them to go pound sand, I'd say opting out of disclosing sensitive information they don't need by not signing in/up and equating their unreasonable demand with a ban is respectable.
