Ryan McCue has posted an update on Make WordPress Core that aims to bring the WordPress REST API plugin to a stable 2.0 version, which would be an official sign of enforcing non-breaking changes from then on.
The team has decided to not propose endpoints for inclusion in WordPress 4.6, and is aiming to propose for inclusion in WordPress 4.7, which based on project history would be released sometime at the end of 2015 or early 2016. The release will also be lead by Matt Mullenweg, who has been the most vocal WordPress Lead Developer about the API, so his buy-in for the endpoints will be very important.
The REST API leadership team (Ryan McCue, Rachel Baker, Joe Hoyle, and Daniel Bachhuber) also want to see core contributors and component maintainers to take over much of the day-to-day REST API core work, ensuring their components are functionally consistent with the API. They see themselves as advisors in the future, plus leads for feature projects within the API, but they can't keep up with mirroring the pace of WordPress core by themselves.
Moving forward, the API team sees our role as advisory over the API itself, with the API treated as an integral part of the component rather than maintained by a separate team. We’re also going to continue to work on our feature plugins (metadata, site/multisite, menus/widgets, and authentication) in parallel, and are looking for help on these as well.
From my point of view, this seems like pretty much the only sustainable way forward for this team. There is simply too much going on in WordPress core for such a small team to stay on top of every change — it's equivalent to maintaining a fork of WordPress. It will be pivotal for component maintainers and other core committers to be involved in making sure their component and the REST API are aligned.