The help center page has a lot of info. It does support sub domain support. I don't use it so I'm not sure it follows the spec you mentioned or not but it seems

The help center page has a lot of info. It does support sub domain support. I don't use it so I'm not sure it follows the spec you mentioned or not but it seems to have a fairly simple protocol. https://support.google.com/domains/answer/6147083?authuser=1 https://support.google.com/domains/answer/6147083?authuser=1