At a certain point, it's probably easier to build and run the unmaintained software in a container running an OS of corresponding vintage. Obviously that has security ramifications, but hopefully this software isn't facing the internet and being in a container at least means you're on a modern kernel and can isolate things at a port and filesystem level.

Debian is super good for this. Most of their apt mirrors are still up and running, so you can easily download and build software from 2002 if you use the appropriate Debian release to do it.

I usually do this first to make sure the software builds and works, then I try to figure out how to get it working on a newer OS.

You're right, but it's also significantly more inconvenient.

it would be nice if they provided a gcc-ancient-compat or something though it's easy enough to create an alias with the correct flags.

-std=gnu98 works. ie explicitly telling it your code is not modern C.

The new flags apply regardless of -std value, as does -fno-common

Uhh no they don't, and it's explicitly documented under https://gcc.gnu.org/gcc-14/porting_to.html in "Turning errors back into warnings".

Futher more every warning Explains why the std value affects them.