Prior to Plasma 5.21, modal dialogs had the close button just like regular windows, but not anymore. The window close button was light/dark, giving a lot of contrast, so it was very noticeable; now, the close button is red, which still gives contrast. However, the new modal design dialog doesn't have a intuitive close button. Personally, I think is hard to recognize the close button. There is too many elements in the dialog and the close button gets lost:

Proposal:
Modal dialogs should have a more noticeable close button at the bottom or have the close button back in the titlebar.