All brushes/pencils are rounded shape. Where did the square shapes go?
Photoshop moved square brushes to a new section called: Legacy brushes.
You can reach it by choosing the brush/pencil option, then hover over the canvas and right click. Click on the upper right corner where the little cog with the small arrow is. At the bottom of the list choose "legacy brushes" and click OK.
The Legacy brushes set will be added to your brushes list. Open them and look for the square brush sets and choose the appropriate one.