I haven’t gone and formally made a ppe yet (though I’ve had multiple characters that could potentially qualify as such), but I’ve never viewed having a backpack as being a threat to ppe status. You’re just able to lug around more loot you’d want to stash without making as many trips back and forth.
If people are worried about you being able to store more swap-out equipment (if you’re that kind of guy), then you could always limit yourself when that arises! But I’m curious to see what others think. I’m guaranteed to be out of the loop with the nitty grittit’s of the “rules” (whoever made them, anyway? Or was it a group of people with a general idea that bickered to some sort of a consensus?)