I'm struggling to understand how any "mass deportation" is feasible. I'm sure we'll hear about it more, but whether it gets into the tens of millions ... I don't think so. I suspect "they" will have some highly publicized deportations of hundreds, thousands maybe, and then it sort of dies away as a thing. Tens of millions? I don't think there is any chance at all.
For one thing, under current law as I understand it, just BEING here is not illegal, even if you over stayed your visa, or snuck on one time. Then there is the issue of determining who is here "illegally", I don't know how they can do that. Sure, for those who ran afoul of other laws, have at it.
I would favor a different approach.