Not unstable per se, but Archlinux has rolling releases. 99.9% of the time it is actually very stable, considering you're always running the latest bestest newest (release) versions of everything, but in rare cases unmaintained stuff stops to work because libraries or environments they relied upon had major updates or changes (which is, as far as I can tell, what happened in this case).
Sventaro, the maintainer of the sfml package, decided to move to 1.99-git instead of completely removing sfml from the official repos, as quite a few Arch people are using SFML. I now think (after having had breakfast and fixing the incompatibilities caused by the changes to Keyboard and Joystick input) that that was the correct thing to do, even if it means some inconvenience.