Folks, this is not a "new law" or anything else. It's right now USCIS musing about not stamping folks at approval time. We have no idea what is going to happen, and I wouldn't get too worried.
Essentially, all the stamp is, is evidence that you have been approved for permanent resident status. It contains your A# and class of immigrant visa, which the POE can then look up in their database. The stamp itself didn't provide any proof on its own.
Once you are a permanent resident you are free to enter and depart the US at will. You will not need AP as permission to return. To be honest, I'm not sure why they bothered with the stamp in the first place - if you presented yourself at the POE with your A# alone that would be all the information they needed to look you up and admit you.