@dansup I think that devs should offload the supported activities on the clients, not on the services side. An activitypub server should accept whatever, and then only the clients will interact/show what they support.
As long as you think in terms of what your AP services supports, you'll end up with needless complexity.
@mariusor Most implementations have a web client that render activities, are you saying they should support other activities (and implement C2S) that may never be exposed to the vast majority of users?
I'd say the issue isn't so much C2S vs S2S, but rather having unified UI components across all clients.