Btw, my usual problem with such new features is that it's impossible
(AFAIK) to use them in a manual in a backward-compatible way,
i.e. while making sure older versions of makeinfo will process the
manual without errors. That's because there's no way (again AFAIK) to
write a condition that tests whether a given feature is available, nor
even one that tests the version of makeinfo that is processing the
file.
This forces any real-world project to avoid using the newly introduced
features completely, until enough years pass after their introduction,
because telling the end users to install the latest Texinfo is not a
practical option.
So perhaps we should introduce a facility for such tests, and
preferably in a way that is itself backward-compatible, e.g., testable
with @ifset or some such.