One morning, my iPhone 11 read “This iPhone Unavailable”

Someone tried too many times to enter wrong passcode.

You can sometimes remedy this without a computer if on iOS 15.2 or later, but you will still need the Apple ID password. It will WIPE the phone, no choice!

How to reset your iPhone, iPad, or iPod touch when you forgot your passcode - Apple Support

If your phone looks like my picture, the blue link is the only solution.

If you have Erase iPhone bottom right, tap that.

(Most people ask the other people in the house if it happened)

Ok thanks.

I have never come across a case like that where a phone cured itself from that message.

In any case the message was only introduced in ios 15.2, so there is obviously a reason that we don't yet understand.

Apple have not instructed the volunteers on here (like me) about that message at all, so we are all learning as we go along.