Yes I think in app browsers should still follow parental controls, and I don't see why that wouldn't already be a thing on devices. If I can use a VPN on my phone and have that block sites for me, it seems like it should be pretty trivial for the phone to respect parental controls across all apps, not just specifically web browsers.

That would be an additional capability. But having to force a website to give specific apps permission to display them in-app seems like a removal. Some people are also suggesting removing in-app browsers which also seems silly.