Another wrong rules I've seen blindly followed is making everything an edge-to-edge canvas, so that the sidebar floats on top. Having a full-window canvas with floating sidebars can make sense for applications where content is expansive and inherently spatial (like say, Figma) or applications where the sidebar is an actual floating element that can be moved around (like Photoshop once was).
It doesn't make sense in Finder, or Reminders, where the content is ultimately just a list. Forcing the sidebar "to float on top of the content" yields no benefit because the content wont ever scroll under it, and because it can't be moved anyway, but it does lead to wasted space, that ugly "double border", etc.