I am thinking about a page on EmacsWiki that holds the
information. This page could for example have entries like
=== Tabkey2.el 21=n 22=y 23=y ===
elisp:tabkey2.el
comment ... comment ...
ELPA could then fetch information from this page. And users could too.
The elisp pages on EmacsWiki could at the bottom have a link
to this page.
A file-header field in each library file can give the same information. I use
this field, for example:
;; Compatibility: GNU Emacs 20.x, GNU Emacs 21.x, GNU Emacs 22.x
However, the field value is free-form at present. A conventional form would let
tools pick up the value.
An advantage is locality of reference and update: some developers are more
likely to update a file header field than a separate wiki page.