There is some activity in the GNU Emacs repository (but not on the webpage of prolog-el package). As I can see there were 26 commits to prolog.el from the beginning of 2014 . When prolog.el was integrated to the mainline GNU Emacs it had version 1.22. Seems no one cared about bumping version of it in the GNU Emacs repository, so Emacs will not object to install in fact older prolog.el from , because it has version 1.25 (still released in 2013). It is misleading for Debian users.
 http://git.savannah.gnu.org/cgit/emacs.git/log/lisp/progmodes/prolog.el  https://bruda.ca/emacs/prolog_mode_for_emacs
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