We are removing the legacy collection endpoint for Editor Interface entities in the Content Management API:

/spaces/:spaceId/environments/:envId/content_types/:ctId/editor_interfaces/:interfaceId

This endpoint was removed from our documentation, tooling and the Web App in early 2016 and replaced with a subresource of a Content Type. Since then it was kept in place only for backwards compatibility. We didn't register any requests to the endpoint in several months and hence are going to deprecate it.