First of all: Thank you for your feedback!
I'm using Unity's default input system, so, I don't think it's a glitch...
This image shows the position of the delete key on a default windows keyboard:
This is the key I have used in Unity's input system.
I'm not sure if this is equal to a mac keyboard or how unity will handle this button if it won't exists, but I think a mac computer should at least have some key combination to simulate this key.
Google says that you will have to press the fn-key + delete key(The one with that you will remove text).
The only bug I know: You can't delete units while hovering the UI with your cursor.
I hope this will solve your problem.