That's impossible by means of how they're written. GNU still cares about operating systems that don't bundle the mdoc macros with their troff processor and don't use mandoc (which is... exactly only AIX and HP-UX at this point), so they write all their text in man so that they render correctly everywhere.

For tagging, the formatting would need to be redone to be mdoc, which breaks both the OSes mentioned above and would be an immensely arduous task at this point. Probably not going to happen.

Note GNU and and the Linux man-pages project are mostly outliers; everyone else that lives in BSD/illumos land has long since moved on to mdoc since like the 80s. See also https://www.usenix.org/system/files/login/articles/141-dzons...