Observers! This is on the critical path towards relations, and one of the star ECS features of the coming release.
I'm broadly confident that these should exist, but haven't devoted an afternoon to a thorough review yet. That's mostly on me, although we'll need another SME-ECS (or Cart, who's effectively an SME-Everything). It gets the new S-Needs-SME tag and stays. No getting nerd-sniped! There's more triage to do!