until recently it worked that you could create a square in Photoshop with the selection check tool by holding down the shift key. Doesn't work anymore! (I'm using the latest Photoshop 2022 version 23.0.1. Is there a default setting that I didn't take into account?
On Windows the Shift constrain Modify key still works with the Rectangle marquee tool. Try resetting your Photoshop Tools and if that does not resolve your problem try resetting your Photoshop Preferences. Tools