The sea level in superflat worlds is currently at Y = -63.

This causes some unintended behavior:

• MC-277216 - Players need to stay below Y=-63 instead of Y=63 to stop Phantoms spawning in superflat worlds
• MC-276319 - Aquatic mobs fail to spawn naturally in superflat worlds
• Changes to the height at which snow occurs instead of rain. In a superflat Taiga biome on 1.21.1 snow would fall starting from y=153 (sea level of 63 + 90), while on 1.21.3 snow falls starting from y=27 (sea level of -63 + 90).

Cause

The change occurred in 24w33a. The hardcoded sea level was replaced with an argument that can be set in the chunk generator. See the original issue: MC-226687

Suggestions & solutions

1. Set the sea level to the highest layer of the superflat terrain when making a new world
2. Set the sea level to y=63 on superflat worlds upgrading from older versions

How to recreate

Create a new creative mode superflat world. Select the Classic Flat superflat world preset and change minecraft:plains to  minecraft:taiga. Enter the world and run the command

/weather rain

Then, ensure you are flying and run the command

/tp @p 0 40 0

In versions after 24w33a such as 1.21.3, it will be snowing. In versions prior to 24w33a such as 1.21.1 it will be raining.

For recreating the phantom spawning and aquatic mob spawning issues see their respective issues linked above.