Looks like it was removed on accident here:
https://gcc.gnu.org/git/?p=gcc.git;a=blobdiff;f=gcc/doc/install.texi;h=3bf2305c8d14181bfb61d112ab3e1c0c2f605322;hp=5735f054317e08b2c5b629adfe72a308459b8bd9;hb=e8645a4001a8d117dd336ea75942aac49101af49;hpb=3825be8c96775cf8e6fcb5eef04455f07717a5ea

But it is not required to build gcc rather only if you are modifying gcc.

Thanks,
Andrew

On Thu, Oct 26, 2023, 11:01 Simon Sobisch via Gcc <gcc@gcc.gnu.org> wrote:

> https://gcc.gnu.org/install/prerequisites.html has a bunch of tools
> under "Tools/packages necessary for modifying GCC", but GNU Bison is
> missing.
>
> I found it interesting to see that some files like under intl say
>     1 /* A Bison parser, made from plural.y
>     2    by GNU bison 1.35.  */
>
> Not sure if that would still be able to be processed with that version
> (but it definitely has some bison3 adjustments).
>
>
> Back to the issue at hand: please add Bison to the list referenced
> above, including its minimal version.
>
> Kind regards,
> Simon
>