Difficulty is dependent on what level you are, and would take the average level of online players in multiplayer

within a configurable distance between each other if possible
configurable +/- level range. ex: say average is level 43, with a config of -5/+10 mobs would spawn between level 38-53)

Added with 143ad2c

Does this include PlayerEx ? I don't see anything regarding it

Nope, only added levelz compat

Are there any plans for playerex ?

Not currently

How about now?

playerex support should be in a different issue, rather than using the same one. it's misleading to see an open issue for levelz support when the mod already has levelz support.

I've added playerex support on the 1.20 and 1.21 version.