Comment 3 for bug 1978543

At the moment 0.0.0.0/0 instead of default should work for network config.

Ultimately cloud-init needs a allow "default" in cloudinit.net.network_state as it will pass it through directly the netplan renderer reserved word "default" in config v2 support.

That said other renderers will not be able to support a "default" value so we may have some renderer impacts as we try to support this "default" value.

Looks like the "to: default" support was added in netplan.io in this commit https://github.com/canonical/netplan/commit/7844058d
which was in netplan version:

$ git describe 7844058d
0.98-129-g7844058

Bionic and later have the following netplan:
0.99-0ubuntu3~18.04.5