The bug

The Stone platform of the Void superflat preset can no longer be toggled on or off regardless of whether the decoration parameter is present after the minecraft:air;minecraft:the_void argument. By default, the said parameter is now absent when viewing the argument from the preset settings.

Even turning off generated structures does not toggle the Stone platform. This is not the case in 1.15.2.

The bug is caused by the fix of MC-184675.

How to reproduce

1. Create a void world with the Void superflat preset.
2. Notice that the decoration parameter is no longer present, and then generate the world.
   → The Stone platform is generated regardless of the absence of the parameter.
3. Repeat the same procedure but with generated structures turned off.
   → The Stone platform is still generated.