Java Edition 1.15 pre-release 3, vanilla / no mods
Slime chunks do not spawn Slimes.
Tried using commands to kill off everything except player and slimes, repeatedly, but still no slimes spawn at all. Not near a swamp so only testing slime chunks.
Also tried clearing a massive area and setting ground to glowstone, so nothing could spawn except for slimes, waited ages but still nothing.
Addendum: Slimes do spawn in swamp biomes. Just not anywhere else.
Comments 2
Superflat worlds seem fine, so I went all out and hollowed out a 200x200 cave from Y=11 to just below the surface. Loads of mobs spawning on the floor but still no slimes. Lit up the area using hundreds of torches, killed everything and stood in the middle. After about 3-4 minutes, I finally got a single large slime spawn.
So it seems that they do spawn, but the criteria are so restrictive that, in normal gameplay, Slimes effectively aren't a thing outside of swamp biomes.
Have you tried loading a Superflat world to see if they spawn there?