I uninstalled 2017 and reinstalled 2018 and all the square brushes are gone. It's hard to even find load brushes because they changed the terminology to Import Brushes. I even imported a 'megapack' but it doesn't have any squares in it.
This still didn't work for me. I hit Legacy Brush and Restore the Converted Legacy Tool Presets Brush Set to the list of Brush Presets? and still couldn't find square brushes.
When you load the Legacy Brushes, it will create a folder in your Brushes panel called "Legacy Brushes". If necessary, collapse the other panels until you see it. Then expand the folder for Legacy Brushes. Look toward the bottom, as they will be in alphabetical order.
If you still don't see them, show a screen shot of your Brushes panel with the folders collapsed.
Does this work?