I am an adult. An 11 year old doesn’t need access to all the things I have access to. It’s my view that 11 is too young for their own smartphone. That doesn’t mean they would be uncontactable when out alone.
OK so you've solved the problem for year 7, say. What about year 8? Do you suddenly give your child a phone with no oversight and no limits? Or do you again say to yourself "no, that doesn't seem very safe, better have no phone at all"? What do you do at the point where you can see that a phone would be useful, but you still can't be 100% sure your child will know how to use it wisely? That 100% certainty that the lines of communication are perfect, that they understand all the issues etc., may never come. But at some point they'll need to start learning how to use a smartphone.
You can give a year 7 child a phone that has no access to YouTube or has time limits on it. You can make it impossible to install apps without your permission, so they get installed at home not during the school day. You can gradually relax those limits as the years go by. Just because an 11 year old doesn't need access to all the things you have access to doesn't mean they can't use a smartphone (in the same way that having their own pocket money doesn't mean they're suddenly free to spend it on a bus into town instead of going to school).
I suspect most kids don't care as much but I have an intensely private DD who hates me looking at even the most innocuous of WhatsApp convos with her friends. I was the same as a kid, just had a vvv low embarrassment threshold.
I understand and to me this is where discretion comes in. You can do a quick check of a phone every so often late at night, and you don't refer to anything in there unless you see something that concerns you (and even then, if you can do it without making reference to the phone, you do).
You also don't have to read the chats with known people - it's looking out for the unknown people, keeping an eye on the big class chats that can get a bit out of hand, and things like that, that are more of an issue.
But again I think it's also really important that kids aren't relying on phone chats with friends as truly private, as that can come back to bite them later - they should be being a bit restrained in online chats, whether it's in anticipation of a parent who might see, or of anyone else their friend might show the chat to.
There is a tension between the illusion of privacy a personal phone offers, and the need for adults to supervise the use of a device that basically puts a young teenager in direct contact with potentially anyone in the world. It is difficult and needs tact and discretion, however you manage it.
To me the smartphone a year 7 child has is more like a portable offshoot of the family PC, than anything like an adult's phone. It can be similarly supervised and limited. Then as the years go by, it becomes more personal to them, they take more responsibility for it, and it eventually becomes nothing at all to do with the family any more.