Description After selecting some pixels, pressing the escape key on the keyboard should deselect.
Additional context
Currently, the best way I've found to deselect all is to use the rectangle or ellipse select tool (press s if it isn't already in use), then click anywhere on the canvas. This feels unintuitive especially coming from other editors such as Paint.NET, MS Paint, and even xpaint which all have ESC to deselect.
I didn't know about the "Deselect All" button at the top until I was searching the issue tracker for something else (I don't use any of the buttons in that section anyway but that is a topic for a different discussion). But I prefer to use my left hand to press buttons on the keyboard so I don't have to break context to move my mouse all the way to the top of the screen.
Tested on Pinta 3.0.4 from Flathub