Forget Me Not

   This is the old version of expand-file-name, before it was thoroughly
   rewritten for Emacs 10.31.  We leave this version here commented-out,
   because the code is very complex and likely to have subtle bugs.  If
   bugs _are_ found, it might be of interest to look at the old code and
   see what did it do in the relevant situation.

   Don't remove this code: it's true that it will be accessible
   from the repository, but a few years from deletion, people will
   forget it is there.  */

Let me check, just how old is Emacs 10.31? According to the Emacs timeline Emacs 13.0x was the first public release - in 1985. Feel free to study the over 30 years old sources, simply because they’re significantly more readable than what’s making up expand-file-name these days.

Oh and yes, there’s of course been fools who tried removing this. The comment itself was added in 2001 after such an incident.