In reply to Summer.

I'm pretty sure "second version of the REST API plugin" on Wordpress.org's plugin repo (https://en-ca.wordpress.org/plugins/rest-api/) is the newer-but-unstable version. It has a lot of improvements but isn't compatible with the earlier version. It's what they're hoping to put in WP core. I think they put it on wordpress.org to encourage folks to test it and get more feedback (also why they posted about this)