When you plug your phone in you are already “bypassing” the battery. It’s just how batteries work, they can’t be powering a device while being charged at the same time since the current can only go one way at a time. Batteries can’t charge and discharge simultaneously. So what happens when you plug your phone into power is that the Power management circuit directly powers the phone and excess power charges the battery.
It’s just a software decision only the people in charge at Apple know why they would limit this to new models.
The battery is not always in the power path. I thought that was how it worked for a long time but got clarification from a writer at anandtech who used to do SoC deep dives. When plugged in the iPhone is directly powered by the wall adapter, excess goes into the battery.