Been a while since I wrote a release update; the 1.4.x line was comprised of various small fixes.
We're up to 1.5.0 because of internal API changes; the execution planner now includes information on argument types (e.g. quoted string vs. greedy unquoted string) which Brigadier needs. This also allows downstream implementors like frame-fabric to support e.g. Item argument types, which need to be handled specially in Brigadier.