Even with OpenBSD that isn't always true. The time_t fixed killed binary-compatibility between 5.4 and 5.5 on 32-bit systems, which meant that any installed packages had to be uninstalled and reinstalled if one were to perform an upgrade. The OpenBSD project isn't afraid to break compatibility if it means fixing a bug - something that I think is a very good thing, but has implications in terms of support.

On another note, upgrade hell is a pretty convincing argument for systems like NixOS, where upgrades can be easily rolled back, configurations are declarative, etc.