Comment by Grimeton

Yeah "You don't break userspace" refers to the kernel <-> userspace interface (libc) because that's the smallest common denominator and the reason why stuff like snap works.

Besides that userspace has been incompatible between distros for at least 30 years because everyone has that one special patch the library needs on their distro that isn't accepted upstream and breaks things beyond repair.

This was already the case in the 1990s when Applixware shipped 30 different versions of their software on one CD-Rom because it should run on different distros and their versions.

And back then there weren't that many, mostly Red Hat, SuSE and Debian.