year ago here was talk that we cannot do upgrade it more, because new Chosen have b/c problem, eg: new class names, new events names ...
and for future would prefer drop it in favor Select2

Yeah we had to take a decision to not go beyond v0.x of chosen because of the b/c break with the class names and the effect that would have on 3rd party components :(

Closed for the reasons stated by @wilsonge

Well, maybe something can be done about the BC problem. Let me come back to it later. Moving to Select2 seems trickier but what if select enhancement wasn't specifically tied to any single solution. What if the JFormList just accepted an option like enhance="true" or something and then whether that enhancement came from Select2 or any version of Chosen was just something handled maybe in a layout or something. Would something like that work?

Hmm maybe. If we left the original version of chosen in for b/c. But then you also run the risk of having two versions being loaded (new chosen/select2 and old chosen).

The problem is also people rely on niche features of these things so in a major version I don't think we can just move from select 2 to chosen 3 (as a random example) say in the middle of the 3.x release. It would just be making it easier for people to transition between Joomla 3 and 4