Some food for thought:

  If GitHub flipped a switch and enabled IPv6 it would instantly break many of their customers who have configured IP based access controls [1]. If the customer's network supports IPv6, the traffic would switch, and if they haven't added their IPv6 addresses to the policy ... boom everything breaks.

  This is a tricky problem; providers don't have an easy way to correlate addresses or update policies pro-actively. And customers hate it when things suddenly break no matter how well you go about it.