The just released FreeBSD 15 for example as a major release is supported until end of 2029, how much more LTS support do you want?
The minor point releases are close to a year in support. And that is only talking base system. Packages and ports you can also easily support yourself with poudriere and others.
As for backwards compatibility: FreeBSD has a stable backwards compatible ABI. That is why you can run a 11.0 jail on a 15.0 host. With zero problems.
Other way around is what doesn't work. You can't run a 15.0 jail on a 11.0 host for example. But backwards compatibility is definitely given.