Squids never spawn in superflat worlds, even during chunk generation.

Steps to Reproduce:

  [Step 1] Create a new world.

    settings: peaceful, creative, superflat

    presets: minecraft:bedrock,128*minecraft:water;minecraft:river
  [Step 2] Log in.

Observed Results:

No mobs spawning at all.

Expected Results:

Aquatic mobs spawn periodically.

Notes:

Occurred after the Salmon Update.
If you start the same world with 1.21.1, squids will spawn.