It seems that it's not so much about pre-1989 "code," but rather Makefiles. ANSI C preprocessors like yours will convert tabs to spaces, and Make doesn't like that. It seems reasonable to let this use case be handled the way it always has been: by using an old tool that still works.

The other major area of difference seems to be that pre-ANSI people used foo//bar to paste tokens (whereas now we use ##). If we're talking about C, that's easy to update; apparently some Haskell folks can't do that for their own reasons. Again it's a use case which is not preprocessing of C or C++, so it seems OK to ignore it if you're implementing a C preprocessor (as opposed to a generic macro expander usable with Makefiles and Haskell).

I wrote a 'make' program in the 80's (I still use it http://www.digitalmars.com/ctg/make.html) and it has a macro processor in it, but it is not like the old C preprocessors.

Warp does support the obsolete gcc-style varargs, but other obsolete practices it discards.

In any case, I haven't seen any Makefiles big enough to benefit from faster preprocessing.