- You can get rid of those annoying pop-up notifications by doing Cmd-Period (Cmd+.) It will work when even Esc / Cmd+W won't! It's a hidden shortcut not many know about, and not on the linked Apple shortcuts page.
- You can make all those system dialogs navigable (to use tab and spacebar to execute buttons) by enabling: System Preferences > Keyboard > Shortcuts > "Use keyboard navigation to move focus between controls".
It can take years to learn everything that macOS secretly offers, but no more than it took me to be at that level on Windows, or any Linux DE. I agree though, multiple Linux DEs seem superior to both macOS and Windows with their out-of-the-box hotkey ability. For everything else, there's Mastercard, uh, AutoKey.
The Command + . was sort of a general "stop" shortcut on Classic MacOS. Most apps with some sort of cancelable operation used it to stop the operation which often caused the modal to be dismissed.
Since old MacOS had cooperative multitasking if an app started some task that never offered control back to the OS it monopolized the system. Many (but not all) apps used the Command + . shortcut to cancel those sorts of tasks.
And for those who'd like to use a keyboard shortcut to toggle keyboard control of system dialogs, there's ctrl + F7.
Although I see this is no longer displayed in the system preferences area you mention, but just tested and it still works on 10.15.6. Wonder if this is the first step in that keyboard shortcut's removal due to the touch bar.
- You can make all those system dialogs navigable (to use tab and spacebar to execute buttons) by enabling: System Preferences > Keyboard > Shortcuts > "Use keyboard navigation to move focus between controls".
It can take years to learn everything that macOS secretly offers, but no more than it took me to be at that level on Windows, or any Linux DE. I agree though, multiple Linux DEs seem superior to both macOS and Windows with their out-of-the-box hotkey ability. For everything else, there's Mastercard, uh, AutoKey.