I had trouble trying to run libc5 era software a few years ago. This wasn't the kernel but the distro did not make it easy. I wouldn't expect them to keep maintaining libc5 packages 20+ years later either.

I think if you do anything GUI-ish over the last 20 years there's been many shared library breaking changes. But that'd be true of FreeBSD too since those dependencies are just the same when they live in ports.