Operating system: Windows-10-10.0.17134 64 Bits
Graphics card: Quadro P4000/PCIe/SSE2 NVIDIA Corporation 4.5.0 NVIDIA 397.44
Broken: version: 2.80 (sub 74), branch: master, commit date: 2019-07-11 13:50, hash: rB06312c6d2db8
Short description of error
Pressing the delete key in the keymap editor deletes the entry even when you are in the "Press a key" mode. So instead of the delete key being assigned to the Keymap entry, the entry is deleted. This makes it impossible to assign the delete key to a keymap in Blender 2.8 unless the functionality exists somewhere else in the interface like a menu.
The keymap for the keymap editor can't be edited as far as I can tell, so I can't change this behavior myself.
Exact steps for others to reproduce the error
Open Preferences editor. Go to the Keymap tab. Find an entry of the "Keyboard" type of event mapping. Click the assignment button and press the delete key. The entire entry disappears.