I definitely can see why you'd see RC3 as a complete rewrite, but I don't understand why you'd feel that way about RC3. There were definitely changes, but they were much smaller and motivated by significant feedback by users who opened issues in response to RC2.

For example, RC2 mandated that all fields were dasherized. We made field names opaque in RC3.

At this point, we're pretty much nailing down tiny details and included this language with RC3:

    JSON API is at a third release candidate state. This means 
    that it is not yet stable, however, libraries intended to 
    work with 1.0 should implement this version of the 
    specification. We may make very small tweaks, but 
    everything is basically in place.

My point is it became an RC3 only a few days ago. If OP used this a year ago it wasn't an RC, just an unstable spec.