This page explains the deprecation policy for libsemigroups, it's not very complicated.

Any function, type, or alias with the attribute [[deprecated]] will be removed in the next major release. So, the alias IsBipartition, for example, will be present in every version of libsemigroups starting with the number 3, but will be removed in the first version starting with 4. The documentation should indicate an alternative for the deprecated entity, in the example of IsBipartition<Thing> this is std::is_same_v<Thing, Bipartition>.