Blender 2.7x or 2.8x added a feature called “Global Undo”
This is not correct; Global Undo is a lot older (the oldest Version I have on my computer is 2.63 and that already has Global Undo).
Undo Legacy was introduced when the new Undo system was added (2.82 or 2.83, I think), because the ‘old’ undo was very slow (but more stable in some cases).
I would not recommend disabling Global Undo. (But of course you can do whatever works for you.)
