Essentially, there are two major changes at play here.
First, when you press jump to fly up and you're going above your max speed, the game starts to normalize your flight speed if you're going above maximum speed. Each time you press jump, you go slower and slower until you slow down to normal maximum flight speed. This essentially nerfs spindash flight, although you can still do it somewhat like a glide, you can't go upwards without gimping your speed significantly.
Second, there's been a fix to a long-standing movement bug in the game that was easiest to notice as Tails. In older versions, when you're moving at or above your character's maximum speed and try to say, strafe right, the game would determine that your input would put you above your maximum speed, and basically drop the input to prevent you from going too fast. This code is why when going fast, you wanted to hit back to slow down slightly and THEN turn, because otherwise the game would prevent your action for going over the maximum speed. This was particularly bad for Tails's flight, because Tails's running speed is faster than his flight speed, just running forward and then flying would trigger this effect. Since you didn't slow down while flying, you'd basically permanently be in this state of going faster than flight speed and having your directional controls ignored unless you deliberately hit back to slow down.
In 2.1, instead of ignoring input when you're going at or above max speed, it adds the movement vectors together and then normalizes them to your max speed. This is the primary reason why Tails's flight is so much more responsive to changing direction now; it isn't dropping inputs when you're going at or above your max flight speed, and you're also losing speed if you're already over maximum, keeping your speed low and easier to control. If you're flying really fast somehow and strafe right, you'll turn right, but the vector normalization will quickly return you to your max flying speed.