It was mentioned before that Phantoms are spawning in Mushroom fields biome and I can confirm it is STILL in effect, but I noticed that Axolotls and Glow Squids can spawn there as well given the appropriate conditions.

1. Start a Superflat world of a Mushroom fields biome with at least 2 layers of water (below sea level) and a layer of a random block above them. Example: minecraft:bedrock,2*minecraft:dirt,2*minecraft:water,30*minecraft:air,minecraft:glass;minecraft:mushroom_fields

2. Notice that Glow Squids and Axolotls can spawn inside the water.