It was being done by a group of snapd developers at Canonical, IIRC, but after a couple of years of exactly zero interaction from anyone outside Canonical I think they just gave up and decided it wasn’t worth it because they were getting accused of trying to monopolise whether they had an open store or just an open API.

Of course, you can also distribute snaps without using the snap store API. I’ve used this for airgapped machines in the past. You can either just grab the .snap file (which is just a squashfs file with a meta/snap.yaml in it so snapd knows how to treat it) and install it with --dangerous, or you can include an assertion file for that snap signed by a certificate that your machine’s snapd trusts and not even have to do that. (Those airgapped machines trusted our own certificate so we could ensure that the snaps came from our CI process and weren’t a developer’s random test snap).