Source compatibility is surprisingly good, binary compatibility is surprisingly awful, even with Slack.

I have some old Linux CDROM games from ~2000 that are nigh impossible to run, and even a lot of really old software fails to run on old (2.x) kernels without recompilation. But Slackware 4.x wasn't particularly good at this in my testing (i started on a slackware-based retro computing distro[0] but my HDD died with no backup before i finished it)

I mean, other than installing an old distro in old hardware, which i do enjoy. I got a CD copy of Mandrake Gaming Edition on eBay with a bundled port of SimCity 3000 that's very good at running old commercial Linux games.

I think there's people shipping ancient glibc with flatpaks to accomplish this today

[0] https://github.com/queenkjuul/basiclinux-lcars