No, what I am talking about is not a combination of keys for PTT. I have PTT set to just a single letter/button. If I have, per your example, my PTT set to the letter 'A'. And say in a game I am sprinting with shift or crouching with control, and then I want to communicate to my teammates, they hear me on mumble but the viewers on stream cannot. It just won't activate the PTT on OBS. I would have to let go of SHIFT/CTRL to get it to transmit. The modifier keys are actively preventing PTT from transmitting.
I haven't tested it the other way around (pressing my PTT then a modifier) yet. I will do that and see what happens. But either way I think it's important that no matter what combination you hit your keys, modifier or otherwise, it should active the PTT within OBS. Unless of course your PTT is actually set to CTRL + A, then you would have to hit CTRL then 'A' (in that order) to activate the PTT in that case.