After the lighting change in 1.18, slimes in swamp biomes are the only mob to still spawn at light level 7 compared to light level 0 for all other Overworld hostile mobs.

How to Reproduce:

• Create a superflat world with the following preset:
• "minecraft:bedrock,127*minecraft:dirt,minecraft:mycelium,minecraft:brown_mushroom;minecraft:swamp;"
• Make sure 'Generate Structures' is set to OFF.
• The ground is high enough to be out of the range of slime chunks, but within the range of where slimes can spawn in swamps.
• Mycelium ensures the brown mushrooms don't break.
• Brown mushrooms emit a light level of 1 which, due to them blanketing the entire world, should prevent the spawning of all hostile mobs.
• Enter the world.
• Enter the command "/time set midnight".
• You should see nothing but slimes spawn, proving they have different spawning requirements in Swamps from other hostile mobs.