The bug

Blocks MC-109251.

When the player dies in midair, from their pov, they continue falling even after the "You Died!" screen appears, which doesn't make much sense. This issue occurs even in singleplayer. Previously, their screen would stop moving once they had died, occasionally leading to glitchy behavior (MC-109251).

Further, if you go in F5 mode, you can see that your player doesn't despawn as it did in 1.16.5. (from MC-225731)

If the player dies in the void, they will fall indefinitely.

Steps to reproduce

Test in both 1.16.5 and 1.17 development versions to see the difference

1. Go in F5 mode
2. Open debug screen
3. Fall out of the world

Your player will despawn at a certain height and will fall no further in 1.16.5 
Your player will not despawn and will fall indefinitely in 21w19a.