Linux userspace is considered unstable. It's not a cohesive system like the BSDs but lots of independent projects all doing their own thing.
The Linux kernel is capable of running binaries compiled in the 90s. Breaking changes in the kernel-userspace binary interface makes Linus Torvalds scream at people on the mailing list.
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/lin...