You can no longer "jump" off mounts, at least not unless the mount is unable to drop you off at a nearby block. For example, you can still jump off if a horse is high off the ground or if the horse is surrounded by liquids.

Steps to Reproduce:
1. Summon or find a horse
2. Ride the horse, preferably tamed so that you don't get knocked off naturally
3. Do not be surrounded by liquids or high off the ground
4. Place a fence and try to jump over it after unmounting. Try to jump off the horse by pressing sneak and jump at around the same time, with sneak being pressed very slightly before. Just pressing space without a saddle would also sometimes work but crouching and then jumping was more consistent and would work with mounts you can order to jump.

The video should help the explanation

Observed Results:
You get teleported to the nearest block and then jump. You are now stuck inside the pen with the horse.

Expected Results:
You jump off the horse, gaining an extra block and a half on top of your regular jump height. This allows you to jump over fences and 2 and 1/2 high walls (or higher with jump boost, since it adds onto your base jump height) after parking your horse. You would not be stuck inside the pen with the horse. This is a very useful feature.

Extra Information:
The height of the mount matters for how much extra height you get when compared to a normal jump from the ground.

Here is how many extra blocks/slabs each mount that I tested gives. The values are at the highest slab and were tested in 1.18.10. This information is mainly here to make sure all mounts don't end up with just 1 block by accident when this bug gets fixed.
Strider: 2 extra blocks
Llama:  1 extra block and a slab (though there was a bug at some point that made untamed llamas give 2 extra blocks instead of 1 and a slab)
Horse and variants: 1 extra block and a slab
Mule: 1 extra block and a slab
Pig: 1 extra block

I think this bug is caused by the following change in 1.16.210:
Players will no longer dismount into liquids (lava or water) from rideables such as boats

The video attached demonstrates this bug