Re: declare-function in files from Gnus (hashcash.el, imap.el)

[Richard Stallman]

    But the point here is that people should take care when changing code
    of packages which are supposed to work with other Emacs versions than
    the one they are integrated in.

It isn't crucial for the files in the Emacs repository to work with
old Emacs versions.  They just have to work in the next release.