Hello, I am attempting to stand up an IPv6-only (single stack) cluster on a dual-stack network, but immediately slammed into sites like GitHub which don't support IPv6. Is there some flannel argument I can pass to allow IPv4 connectivity to the public internet, or do I truly need to have the cluster be dual stack to reach those sites?

There is no way to magically connect to ipv4 address with ipv6 no, any more than udp can talk to tcp. You could deploy a proxy that is available over ipv6 but also has ipv4 connectivity?

I was hoping there was some support in Flannel for NAT64, but I guess the answer is no

You'd need DNS64 to handle that as well I believe. Easier to use a proxy.