I was under the impression that as long as GitHub doesn't support IPv6 it is a sign that they still haven't finished their migration to Azure. Azure supports IPv6 just fine.

Supports IPv6 just fine? Absolutely not, they have the worst IPv6 implementation of the 3 large clouds, where many of their products don't support it, such as their Postgres offering. See https://news.ycombinator.com/item?id=44881803 for more.