Keyboard shortcut for "Revert Changes" in closing dialog?
Your screenshot doesn't seem to show it, but for me [Revert Changes] has a blue outline...



...which means it can be activated with the Space Bar.

I literally just figured this out (even relative experts can learn something new every day!), but apparently it's been a feature of macOS for quite some time.

(BTW, to change which button is outlined (and therefore which button is activated by the Space Bar), use the Tab key.