My understanding was that the TZ database reflected what users are actually doing, rather than what government policy is. Has that changed?
No, but the TZ database only cares that there exist a set of people/places who use Mountain Time with DST (America/Denver) and a separate set of people/places who use Mountain Time without DST (America/Phoenix). It has never really concerned itself with where any borders between those regions are, no matter how simple or complex.
However, if there were a historical case where a region once observing DST decided to stop, or a region once not using DST decided to start, then that would be codified as a new zone, since the rulesets would differ from both America/Denver and America/Phoenix. But perhaps if only a small handful of people are affected by these effective border changes, it may be too insignificant to bother.