Bind to '::' and you can get v4 and v6 traffic on one socket.
Doesn’t work for loopback (you have to listen externally )
It works fine for me on NixOS (Linux), with a recent kernel version and no weird config options - or at least I think so.
Not on all OSes.
And it's not all good to have one less socket, there are still issues with people not expecting v4 traffic on a v6-capable socket, like described here: