shouldn't stability x197 be a synonym of it? It appeared 183 times
out of 197 without it. In other words, the grouping meant to be
achieved by the tag system, has been broken here.
I don't agree with the premise of the question. ("It" apparently referring to the flight-dynamics and/or flight-mechanics tag.) I'd say that the failure that has taken place here is not that stability is a synonym of some other tag, but rather that users have simply failed to select a broader "umbrella" tag such as flight-dynamics and/or flight-mechanics, in addition to the "stability" tag.
Stability should definitely not be viewed as a synonym of "flight-dynamics" or "flight-mechanics". The "stability" tag needs to be kept, regardless of what happens to the "flight-mechanics" tag. It is useful to be able to find questions that only deal with stability and not with other aspects of flight-dynamics and/or flight-mechanics. And yet this is in no way indicative that the flight dynamics and/or flight mechanics tags wouldn't serve a purpose, if properly used (i.e., if used more often.)
This answer is not meant to weigh in on the question of whether or not "flight-mechanics" is adequately covered by other tags. Just was concerned that the discussion might be turning in a direction that threatened the "stability" tag.
The "stability" tag is certainly a subset of the "flight-dynamics" tag (and probably also a subset of the "flight-mechanics" tag), but losing the finer-scale categorization afforded by the "stability" tag would be a mistake.
Here is an example of a question that should be considered within the scope of the flight-dynamics and/or flight-mechanics tag-- arguably both-- but is not really strictly a question about any aspect of "stability", since the aircraft is simply presumed to be in a steady-state climb -- Does lift equal weight in a climb? The point is a little bit obscured because the asker of the question does go into some detail about what would happen if the forces were not appropriately matched to the flight path, which does touch on stability dynamics-- but the question is answerable without getting into that at all.
Here's another question about forces, where the fact that the aircraft is in a steady-state situation is simply a "given", without getting into "why"-- What produces thrust along the line of flight in a glider?
A question about stability, on the other hand, would look at the balance of torques generated by the wing versus the tail, or would look at the "phugoid" oscillation generated when lift is temporarily larger or smaller than it "should" be for the instantaneous direction of the flight path, etc. All of which would be a subset of flight-dynamics and/or flight-mechanics.
It has been suggested to create a new tag labelled "steady-state". The current answer is not weighing in on that either for or against (though that would seem to go against the guidance here (" Even if you have sufficient reputation, you should only create new tags when you feel you can make a strong case that your question covers a new topic that nobody else has asked about before on this site."), but regardless of whether or not that is done, the current answer supports keeping the "stability" tag as a useful tag.