There’s always been two button modes. “mouseup” is indeed the most common. However, “mousedown” is used in certain cases where the feedback is immediate, for example in the phone app where the number shows up as soon as you tap. However, notice that the lock screen uses neither of these, but rather a strange combination of both: it registers on “mousedown”, just like the phone app, showing a new dot in the field, but then will “unregister” if you move away and lift your finger, removing the dot. As far as I can tell, this is the only place buttons work this way.