@matigo People are using it where they can. Still fairly niche, but unless there's a way of displaying the results, not much point. So, You just replied to my recent post on my Known site. With webmentions, you could have replied from here. But you wouldn't do that easily unless you were also reading that post here. But I don't cross-post to here, because there's no easy mechanism that would give me any benefit.
One of the most interesting developments in that space now is the emerging collaboration between WebSub and MicroPub, to create feed readers that allow you to interact directly with what you're reading. So you can bookmark, like, reply, quote and so on.
Webmentions are the glue that sticks all the bits in all the sites together.
10C isn't getting them because 10C doesn't do anything with them.