WordPress still has those, or are they disabled by now for new blogs? I get them from WordPress blogs sometimes instead of the nicer trackbacks, same thing without xmlrpc. Webmentions are an orthogonal newer system, basically incompatible trackbacks.
Serendipity implements all three now, so there are definitely still blog engines that support these mechanisms.
New internet rule: When the name of a service/software is a common word in any language that already has a meaning, we must escape it if the context is lacking that would indicate it’s title property.
Examples: /Serendipity, /Sheets, /News /Numbers, /Files, /Drive, /Translate, /Play, etc.
Exceptions: If the context is clear, i.e., in a text talking about Google assets where News, Sheets, Drive, Play, are mentioned; or one prefixes the context with “The software Serendipity…”.
We should not assume everyone in the world knows every single title of every single software and service.
Well, context. We are talking CMS or blog engines, and "blogengine Serendipity" does find it as first at first place, so does "cms serendipity". But for the log, I was talking about https://docs.s9y.org/.