Adam D. Ruppe:

The older std.c is kept around just for compatibility with the old names before the move, at least as far as I know. Maybe they haven't fully deprecated it though because there's other reasons I don't know about, since it has been many years now since the move.

In D/Phobos/Dmd there is a ton of stuff that's supposed to be
obsolete, that should not be used, that is deprecated, etc. I
presume Walter thinks that the problems caused from keeping it
and from unwanted usages of it, is smaller than the breaking
troubles caused by removing it. I am person that likes to keeps
things ordered and clean, so I prefer to remove old stuff after a
suitable deprecation period, instead of keeping it around almost
forever (like floating point special comparison operators).

Bye,
bearophile