I thought I replied to this, whoops. To my knowledge the old API is still in use, though the switch to full-HTTPS may have broken some stuff that uses it but doesn't use HTTPS.

We're using the Swagger/OpenAPI spec (Swagger is the old name, and now mostly refers to the toolsets) - it's got an active development community, and a lot of tools for interacting/developing stuff that conforms to that spec.