A Services mailing list would provide useful a forum for design discussions, newbie questions, proposals, prototypes, etc. that are hard to fit into Phabricator tasks or Mediawiki RFCs.
@Aklapper: Your link is a public list we created a while ago, but never announced.
The google group @Jdouglas mentions is the internal team alias (email@example.com). For the public list part I actually like the idea of using phabricator watches instead of a mailing list, as that lets participants choose what project (or even aspect of a project) they are actually interested in. See T77228. For general / newbie discussion, using the established mailing lists (wikitech and ops mostly these days) gives us wider exposure. Phabricator should also work for this, and has the added bonus of being arguably more discoverable via google.