Pretty much the title. To reproduce, create a "redstone ready" superflat world on normal difficulty, and watch the slimes spawn on. This seems like an unintentional side effect, since redstone ready worlds are supposed to be devoid of such spawns.
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No, this is preset specific. Slimes didn't spawn in this preset prior to 1.18

Closely relates to MC-215423.
Duplicate of MC-141262