They are removing both implementations(the new and the broken one) in order to put more work and review on the new one, and release it properly at a later time.
Windows authentication absolutely uses Kerberos, NTLM is more used as a fallback. Kerberos is way more secure than ntlmv2 and is the favored authentication method.
Can't GitHub add a popup asking, "We detected you are in a restricted country, are you sure you want to login?" or just block logging in without locking up the account?