Apps take screenshots all the time, e.g for crash reporting. Then they phone home with them. Most apps obviously ask politely when this happens but I’m sure there are exceptions. Not to mention malicious apps. There is no real security or isolation for screenshots that I’m aware of so app one will happily snap a picture of app two, without needing special permissions. That other app can be your password manager or baking browser tab. So apps explicitly opting into being in the picture is perhaps not such a bad idea.