The bug
The command /spawnpoint
allows for players to set their spawnpoint to a set of coordinates that are out of bounds. If the player dies afterwards and attempts to respawn, the server crashes.
If the player exits to the title screen while they are dead and the world saves, they will no longer be able to use the world as they will be unable to respawn without crashing the game.
How to reproduce
Use the command
/spawnpoint @p ~ ~ ~30000000000
Kill yourself using
/kill @p
Attempt to respawn (you may need to click "Respawn" twice)
This issue also affects 1.16.5 as well.