How exactly are you disabling cliff generation, i figured out the trees, rocks, and water. However, I can not find anything about cliffs, so how are they being disabled?
Just really curious, trying to figure out how to make my own mod.

I don't know how jblaske is doing it, but if nothing else you can catch them on chunk generation and remove them there.

I actually stumbled into it when i was trying to make sure i got rid of all the rocks and trees. during data-updates.lua i'm just killing the autoplace values for any simple entity, thus preventing them from being added to the world.

for k,v in pairs(data.raw["simple-entity"]) do
    v.autoplace = nil
end

That's remarkably simple.

And I'd guess it's far more performance-efficient than going through every entity on chunk creation.

Yeah def much more performant, i was pretty happy when i discovered it.

By setting "simple-entity" to nil it removes rocks and cliffs, correct? Because when I mess with the code and disable it, rocks do spawn again, but cliffs still don't spawn. I actually would like to figure out how to get cliffs back at least.

You will need to look at how the prototypes are created for all of the cliffs/rocks and figure out something they have in common to know which ones you want to kill the autoplace on, and which ones you want to leave it enabled.

That should all be in the base game data files.