[dudus]

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

[teddyh]

> Google Domains uses the dyndns2 protocol

So, not RFC 2136. No standard protocol for you.