They miss out on the social group and then fade away from it and just become "that one guy in our class."
The last time I mentioned this several people argued that, "true friends would stick together" or some such. Well, if you already have those friends. But if you're in high school and finding yourself, you probably haven't met all of them yet.
A lot of both communication and organizing of social events happen through the phone. Kids without a phone (or some online method) will just be forgotten. This is just the reality.
Unrestricted access? That depends on the kid. We had them charge in the living room (no overnight use), and their computers were actual desktops in a single office in the house.
We never used filtering or tracking software. The one exception was blocking youtube (through /etc/hosts) for my youngest during covid when it was too big a distraction.