@evan I assume you are referring to the differences between https://api.zuora.com/rest/v1 and https://rest.zuora.com/v1
As you point out these are two different endpoints for production, however, one does not exactly replace the other so to speak.
The https://api.zuora.com/rest/v1 endpoint is the original REST API that has been around for years and contains the traditional REST API.
The https://rest.zuora.com/v1 is the newer REST endpoint that also proxies the legacy SOAP API into it, thus it has the various APIs under the "/action" path. You can think of the new endpoint as a multiplexer for the various APIs as it accepts requests and routes them to the appropriate backend. (i.e. legacy SOAP vs traditional REST, etc). You can read more about this here: https://community.zuora.com/t5/Engineering-Blog/Moving-Fast-from-SOAP-to-REST/ba-p/16160
The point is, any APIs we had developed against https://api.zuora.com/rest/v1 continue to operate as expected, however, newer paths such as "/action/query" are only available under the new https://rest.zuora.com/v1 endpoint since that has to be routed to the SOAP API internally and is a "newer" REST API.
All that being said, the differentation between these two endpoints could do with some documentation to back it up (other than my observations)
DISCLAIMER: This is all from my exploration and testing of the platform and I have no internal knowledge of Zuora's infrastructure.