If I think of square checkboxes I think of sharp corners. Maybe I should write an article "In Loving Memory of sharp corners" because I definitely miss them but they seem to be extinct from the modern Web and GUI in general. If I skim the article only old screenshots contain sharp corners. None of the new once.

I have a browser where I configured "border-radius: 0 !importent" as userContent.css for fun. Was sometimes surprised how much it is used. Especially how many circles are today actually boxes with a large border radius.