It would be nice if the GNU folks published the emacs info with a version number.

Actually, they do, sort of, since Emacs 27:

electric-indent-mode is an interactive compiled Lisp function in
‘electric.el’.

(electric-indent-mode &optional ARG)

  Probably introduced at or before Emacs version 24.1.
  ...
[download]

Note, however, that Probably indicates that this is based on guesswork (grepping NEWS files). We had cases where it was off by two versions, and the Emacs maintainers made clear that it is extremely unlikely that they'll spend the effort to make that exact.