OpenBSD can do this and get away with it since its use and deployment is minuscule compared to other systems.
- Having a regular and well tested release model.
- Having a clear policy of supporting only the current and previous release.
- Being a BSD style OS where the kernel, libc, and base programs are developed and released as a whole.
- Not making promises about binary compatibility across releases.
That said, they'd be the first to tell you that if their model doesn't suit your needs, you're free to use another OS. They build and support what they want. If you want something else, they'll ask you to get involved and do the work.