That's not possible I guess. Because they aren't able to hold redstone items as I know. and on the other hand, there's no chance that they will spawn on slabs, right?
World Type: Default(was generated in 1.16.5)
No additional Generation Settings
Seed: 3346373486014953634
I don't remember the coordinates / dodn't list it before.
I used chunkbase to find one and I successfully found one. Only the most nearest mansion was missing. and I also didn't get a pillager outpost on my first try using /locate command. The second time worked surprisingly.
The droppers and observers are still there by the way,
I'm using vanilla.