Some cool details here: https://en.wikipedia.org/wiki/Desk_accessory
Like Tahoe, it was deliberate and there's an explanation for the difference.
But I do wonder if people at the time felt the same way.
I particularly like this Bill Atkinson tidbit at the end:
Bill Atkinson complained to me that it was a mistake to allow users to specify their own desktop patterns, because it was harder to make a nice one than it looked, and led directly to ugly desktops. [...] So he made MacPaint allocate a window that was the size of the screen when it started up, and filled it with the standard 50% gray pattern, making his own desktop covering up the real one, thus protecting the poor users from their rash esthetic blunders, at least within the friendly confines of MacPaint.
(He was totally right, making your own desktop patterns was fun but the standard checkerbard was far and away the best choice.)
Article author here. I think the quoted claim is somewhat misleading. There are at least two different ways to interpret a UI feature as "not new":
1) The feature has been in the operating system all along.
2) Something analogous existed 40 years ago and then disappeared long ago.
You're referring to 2, not 1.
The only reason I chose Calculator app for my screenshot is that its window is very small, which allowed me to make a small screenshot, because people may be reading the blog post on small phone screens. In other ways, admittedly, Calculator is not a great example, because its window is not actually resizable, and thus it's not the type of window that you would normally place in the corners of your screen, like a resizable document window.
Rounded corners on a "widget" type of app are not as objectionable. As other commenters have noted, the calculator in "classic" Mac was a special Desk Accessory. In contrast, on Tahoe, the varying corner radii affect ordinary document-based apps.
Consider Mac OS X 10.0 Cheetah. The top of the windows had rounded corners, but the bottom did not! https://512pixels.net/wp-content/uploads/2018/08/10-0-Cheeta...
TextEdit, for example, did not start to have rounded bottom corners until Mac OS X 10.7 Lion, which was itself much maligned for bringing the iPhone UI to Mac.