Today they just announced V1.0 of their backend:
"This Backend Server supports all documented and undocumented endpoints that were originally available on Atom, and should support all interactions with APM, PPM the Atom application, the Pulsar application, and any other unknown 3rd party services that integrated with the backend. Essentially it should look like nothing has changed."
https://github.com/pulsar-edit/package-backend/releases/tag/...
So that's progress.