I have a tk.Button(..., command=_on_button_click).
Once it is clicked, I want to know whether the Shift key is currently held down.
But the _on_button_click is called without any event object or similar for me to check it
Assuming a button named button and a handler named shift_click, you can use the bind method:
button.bind("<Shift-Button-1>", shift_click)
Of course, you will probably also want one without shift click:
button.bind("<Button-1>", not_shift_click)
tkinter check if Shift is down when Button is pressed