It does more than you think it does. GitHub may not support IPv6 and Apple can't force them, too, but the GitHub App on iOS is in most cases using DNS64 to get an IPv6 address to a NAT64 gateway to the GitHub IPv4 addresses, and all that just works today, in part because Apple forced apps to support "I'm always going to give you an IPv6 address, even if you know your own service isn't going to naturally return one".