I would guess a surprisingly large percentage of smartphone users aren't aware this is an option.
Exactly the same here and I am sure any privacy-aware person will do the same.
That really needs to be the default.
On Android, it asks you when you set your passcode for the first time.
Super fragmented, but Nexus 5X had it.
My Sony Xperia also asked me at lock screen configuration. I could choose either way, but could not ignore the question.
Galaxy S7 asks you too. I remember now that you said it.
Agreed. It's usually buried in the device settings and also in Messenger and Hangouts. Signal gives you a few different options as well. But it should be more visible to all users, not just those of us that dig through settings.