A surprisingly large amount of code still works. If your code is statically linked the syscalls have not changed, due to Linux's backwards compatibility guarantees, and if it was dynamically linked against glibc the glibc compat may work, or you can just use the old glibc and linker.
How much useful code there is that cant be recompiled I dont know, but I am sure people have some, and they probably dont want to run it in an old unsupported distro.
