OpenBSD rather notably and explicitly lacks any notion of "LTS" (all releases are unsupported after a year) and is perfectly happy to break things without much

OpenBSD rather notably and explicitly lacks any notion of "LTS" (all releases are unsupported after a year) and is perfectly happy to break things without much of a grace period (prime example being the switch to a 64-bit time_t for all platforms). That said, the upgrade process (like most things) is thoroughly documented every release, and a given release will continue to chug along for years, so I can see where you're coming from there.