Artboard tool does a lot of things: selects, creates, moves,.. we are out of modifiers to control it properly. Specifically, we can’t create square artboards with the natural way we can create square paths, circles with Ellipse tool, like in many applications.

Instead, Ai uses Shift key to select artboards if you start holding Shift before clicking (which is fine) and uses ratio of the last selected artboard if you hold it while dragging (who even uses it?)

What if you let us to create square artboards, when no artboards are selected, but Shift is held? The alternative is to get rid of proportions lock completely.

Please also take a look at the related request: https://illustrator.uservoice.com/forums/333657/suggestions/34561711