This feature exists, on systems that use mandoc as the default manpage formatter and viewer (such as OpenBSD).

In HTML output, definitions for flags, environment variables, and other such things are converted to hyperlinks, and hyperlinks can point directly to them. https://man.openbsd.org/ssh#D

In terminal output, this same capability is used to create a ctags file, which is supported by less(1). On OpenBSD, I can type “man ssh”, then :tD to jump to the tag named “D”.

This relies on the mdoc(7) language’s semantics, so unfortunately it won’t work for manpages using the older man(7) macros. But nearly all BSD manpages are written in mdoc(7), and a significant minority of manpages from packages.