Ask the people who say you are only allowed to use stable libraries what they understand by the term "stable". Sure there are libraries that push changes to master everyday without extensive testing, but SFML isn't one of them. Little gets broken through a commit to master, and if it even happens, it usually gets fixed within a day.
Judging by what your post implies, you seem to think that any revision with a version number stuck onto it is "stable". Version numbers are just like git tags. They just specify a specific commit that you can use to refer to instead of providing the whole hash. They also come with pre-built library files on the website and probably also up-to-date tutorials. However, all of that doesn't mean you can't create any of these yourself for any given revision. You can build yourself and generate the documentation yourself, so there really is no difference between a tagged revision and any other revision besides the fact that you might end up having to do less work in the former case.
If, on the other hand, you are afraid of API breakage, then I can assure you this will not happen within the same major release, i.e. 2.x. The next API changes that might and probably will break code will come in 3.x, but that is still far away.
