FreeBSD isn't too bad, you can build/install compat packages back to FreeBSD 4.x, and I'd expect things to largely work. At previous jobs we would mostly build our software for the oldest FreeBSD version we ran and distribute it to hosts running newer FreeBSD releases and outside some exceptional cases, it would work. But you'd have to either only use base libraries, or be careful about distribution of the libraries you depend on. You can't really use anything from ports, unless you do the same build on oldest and distribute plan.

At Yahoo, we'd build on 4.3-4.8, and run on 4.x - 8.x. At WhatsApp, I think I remember mostly building on 8.x and 9.x, for 8.x - 11.x. The only thing that I remember causing major problems was extending the bitmask for CPU pinning; there were a couple updates where old software + old kernel CPU pinning would work, and old software + new kernel CPU pinning failed; eventually upstream made that better as long as you don't run old software on a system with more cores than fit in the bitmask. I'm sure there were a few other issues, but I don't remember them ...