@rozzin You provided URL for one of more than 20 APIs (actions/requests...), which should work properly in order to tell that "it is supported".
Last year I created the task for AndStatus to "Use Activity Streams API for GNU Social", please read: https://github.com/andstatus/andstatus/issues/378
But after initial discussion, including here in GNU Social, I've got that actually no full-featured client exists that uses ActivityStreams json APIs and hence it would be naïve to assume that the API "works". No GNU Social developer told me: "Just start implementation from your side, I will add and fix, what will be needed, at the GNU Social server side".
In fact, very annoying bug / deficiency in the "search" of the "supported" Twitter-compatible API stays not even commented by developers for 7 months https://git.gnu.io/gnu/gnu-social/issues/206
I guess now you understand, why I think that ActivityStreams json format is not supported (cannot tell about AtoPub...)
@kris