Comment 1 for bug 2046490

While the supplied config is valid netplan, it is not valid network v2 configuration:
https://cloudinit.readthedocs.io/en/latest/reference/network-config-format-v2.html

In order to render to ENI, valid v1 or v2 configuration must be used.

There are plans to add `to: default` v2 configuration syntax in the future. When that change happens, it will be backported to currently supported Ubuntu releases. Additionally, if the `gateway4` syntax is removed, it will only happen in a newer development release once `to: default` is supported, and that change will not be backported as it could break currently deployed installations.

Since the behavior is working as intended, I'm going to change the status to invalid, but if there are additional questions or concerns I didn't cover, we can discuss it here and I can reopen the issue if needed.