I’m not not a lawyer at all and I have no real idea. My guess is that existing visas are already printed and henceforth there is no petition anymore, you have a status or visa already. Since the old petition didn’t require the payment, you don’t need to show proof of payment now. But lol if I actually know how this works, can be anyone’s guess.

My guess is that if this goes forward new h1b visas petitioned while the worker is outside the US will have a line saying “must show proof of payment” or something like that, while petitions while in the US won’t have that line on their visa stamp