In Blender 2.8, default setting for deselecting elements is now ALT + A (former Blender 2.79 and earlier hotkey for playing animations which are now played with Spacebar per default)
Correct duerer
You also have the option in the Keymap settings to enable the 'A' key to toggle the selection (same as 2.79)
Thank you, Wayne, for the hint. I personally find it clearer in Blender 2.8 and "Spacebar" is also the hotkey for starting a video in many video players. Also, deselecting with ALT + A avoids possibly selecting a whole bunch of objects if you just want to be sure that nothing is selected. There seem to be a lot less shortkeys now in Blender 2.8 but it's more than equalized by the cleaner structure.