On Unit Removed Event?

Is there something like this, some system perhaps?

Um, not sure i understand the question. Could you rephrase it?

Can I detect when unit is being removed from the game?
Either by using RemoveUnit() or after it dies and decays and everything...

Simply kill it THEN remove it, which will register in A Unit Dies events.

And what if it is already dead?

nwm...

My system for cleaning removed units, commented, just run in game, also you may use more advanced options, map free to edit.

Isn't it just "A unit leaves rect - playable map area"? Dead units are still in the rect so this would be when units are completely gone.