Linker limitations on 32-bit architectures

Just FYI there already was a first step towards modernizing what autoconf can support... for example configure scripts generated by autoconf these days definitely DO use shell functions. That's been true for >10 years, since autoconf 2.63.

As far as supporting older systems, some of that depends on the software. Some GNU facilities make a very conscious effort to support VERY old systems; this is particularly true for "bootstrap" software. Others simply make assumptions instead, and don't add checks for those facilities into their configure.ac. It's not really up to autoconf what these packages decide to check (or not).

Also, much modern GNU software takes advantage of gnulib which provides portable versions of less-than-portable facilities... sometimes it's not a matter of whether a particular system call is supported, but that it works differently (sometimes subtly differently) on different systems. That's still true today on systems like Solaris, various BSD variants, etc. even without considering IRIX.