Do you want to cripple the entire app industry? Apps built using React Nativ / Flutter e.t.c use the WebView to render themselves. So they’re basically already running “in-app browsers”

But then how do you differentiate when the app is rendering its own view rather than another website? You could apply some restrictions like <iFrame> has nowadays where you need extra security privileges (I think) to render pages / execute scripts not on the same domain

Otherwise you can always open safari from all of these in-app browser views and they could implement a toggle which forces all of them to be opened in Safari automatically