I'm not really seeing the benefit of putting this in the HTML specification. I see the use case, but seems to me this should be left to libraries.
W3C isn't just HTML: they also publish the specifications for CSS, ARIA, XML, XSLT, XQuery, XProc, RDF, SVG, WOFF and so on. Some of which (like the XML-related stuff) have no real bearing on the browser or front-end tech.
We'll be stuck with these redundant proposals for decades. Webmentions could be a fantastic library on its own, though as previously mentioned, we already have trackbacks. Does it need to be an additional few files in the codebases of Chromium, Firefox, et al.? No.