It is not that levels >12 are verboten or anything, it is simply that most people admit that the math gets rather unhinged past level 15 or so and that the vast majority of games end at or before 20.
I don't see the math getting unhinged there. What does that mean? Is there a limit on what DCs you can beat? In the Rise AP there are checks up in the 50s way before level 15. I don't understand it.
In addition the fact is that it take a very long real life time for most games to reach level 20. This means that if level 20 was where discussion and focus was for the game you would have to wait a very long real life time before playing the "Real" game.
Pathfinder has been released for about half a decade if not so. Do we have to wait longer?
Another factor is that at 15+ you run into the WBL issue where any sufficiently determined and system wise person can use and abuse WBL to create a very powerful character out of anything up to and including a commoner just by manipulating gear load-out.
This implies to me we can't discuss the weakness of the system and can only talk about the good as if Pathfinder is untouchable to critique yet I know that's not true given other threads. What makes this aspect of the game unique? That commoners can do it? This would seem to put the impetus on Paizo to refine their item design yet they seem to get a free pass. Why?
Lastly most level 20 builds/abilities/effects do not bear any resemblance to or extrapolate well to what the same character could do at lower levels.
So we ignore them?
All of this make level 20 a place where YMMV is very much a given and so most people will not choose to base their statements off of this rather difficult to define ground.
Is it difficult because the game is somehow more complex (I'd argue it's not; just more of the same) or because there is a culture that says 'we don't talk about it' which results in a sort of communal atrophy in the ability to work in that space either by communication or practice?
Please understand that I believe that high level play is both fun and possible, but do to the increased need for GM intervention, read "Fiat", as you get closer to 20 and past it, it is not the best place in my opinion to discuss or define ideas or concepts especially concerning balance of any kind.
Again, why? Would it not better serve the community to hash out seeming tricky bits to create a shared sense of what it is what? I don't mean to sound like there can be an organized body that hands down rulings. But, balance issues abound in lower levels. I can TPK a level 1 group with a CR 1 swarm. How do we deal with that? We know how to deal with that! And, it's pretty common knowledge too. Higher levels should be no different.
This is of course all just my opinion but I hope it helps.