@elomatreb @rick_777 A few projects do implement it but most of those that do only have a couple of users.
The C2S API is easier to implement the S2S API. So much so that at one point when we were running out of time in the SocialWG we considered *only* publishing the C2S API because it's such an easier deliverable. The main reason it hasn't happened is that the S2S API became popular and the C2S didn't. Inertia.