In System, we have this new endpoint since THREESCALE-4528 & THREESCALE-6310
It is documented in our ActiveDocs:

We need you to use it instead of /admin/api/services/proxy/configs/:environment to be able to remove this endpoint for our improvements in the performance of all proxy configs endpoints/requests (THREESCALE-6237).
Careful not to confuse this old endpoint with /admin/api/services/:service_id/proxy/configs/:environment. That is another endpoint and it keeps working.