At the apex (of the head, not the hand) the head is stationary, rather than rising or falling, and the eyes can best focus on the basket and judge location. An apex shot gives the most time to arrange hand-brain-eye coordination, this is best for accuracy.
At the apex of the head, the arm is still straightening/lengthening and imparting forward momentum to the ball. So the hand is still on its way up, even though the head has reached the apex of its jump.
You can get a bit more power and extend your range if you shoot while the body and head are still moving upward, but with a big cost in accuracy. It is unnatural to shoot this way and I have not seen anyone who does this. Bad shooters tend to shoot when the head is on the way down, good shooters at the apex of the head movement.
I think the historical change is not about the timing of shot relative to jump, the shot always is controlled at the apex of the head movement -- that is, the moment shortly before release when maximum force is applied to the ball and direction of the ball most influenced is also the moment when the head is highest and slowest moving. What has changed is that people don't jump as high and they don't extend the arms as fully before release. Speed of release and controlled accuracy has mattered more than height of release, with the shift to 3 point shooting.
Things happen so quickly it is hard to see the timing in real time. Slo-mo is needed and you'll find that for everyone, the shot leaves the hand very shortly after the head ceases to rise.
429 makes good points about the difference in shooting off the dribble and shooting off screens.
It's hard to understand exactly what you're saying because you aren't quite using the terminology I normally hear. When people say "apex" they are always referring to the apex of the jump. When someone wants to refer to the hand, you're referring to a "set point" (by the eyebrow-ish) or a "release point" (elbow above the eyes).
Your third paragraph really isn't correct though.
There have been great shooters shooting before, during and after the apex of the jump. After the shot seems to mostly be great mid-range shooters because they like to fall away.
While it's correct you get more range when you release on the way up, it's not correct that it impacts accuracy. Literally the best shooter in the history of the game releases before the apex of his (very small) jump. The best shooters in the game right now almost all shoot on the way up.
I'll point back to simple physics. If you're at the true apex of the jump, that means the entire shot is arm-based, because you are no longer moving up or forward at all. The legs lifted you into the air, but they aren't propelling the ball forward and up. That's what you see with
Ray's shot, and that is why he had such a strong arm heave. His shot is still gorgeous, don't get me wrong, but it's just not a shot that is conducive to either shooting off the dribble or from longer distances.
This video explains it via Booker's shot pretty well.
The biggest issue with Ray's type of shot, especially for younger shooters, is that it impacts accuracy at range. When you have the legs involved in the actual propulsion of the ball more (and the same way every shot), you can use less arm muscle and maintain the integrity and consistency of the shoting form. It's how a guy like Curry, Klay, or Booker can seem to shoot the ball for 35 feet with consistency and older school shooters didn't even bother to shoot from there.