No Appending comes with its own bugs. Let’s imagine you’ve opened 4.3 for the first time. You have no other brushes. Select an essential brush and duplicate it


Save it, and you should get a “Saved/Brushes” folder generated in your Assets folder.

If you have any custom brushes you’ve downloaded or created, you can literally drag-and-drop their brushname.asset.blend file into that folder – not in Blender, in your OS. Quite blender if it’s open; restart, and those brushes should be available.