I am unable to get slimes to spawn outside of on swamp land on the Caves and Cliffs Preview Data-pack on 1.17. I started at y-59 and noticed they weren't there so I went back up to y0 and tried all levels to y7 without results. 

Seed: 9035824829380937130
X-176 Z 208

X-160 Z 224

I even went into creative mode on a new world with the same seed and removed tons of  blocks in other chunks and didn't get any slimes to spawn.