In primary school there's been a lot of times where a teacher wanted to show something, but the whole thing was blocked. Super inconvenient. But some students already figured out how to bypass the block easily. Inconvenient, and useless.
In computer class in middle school, it was not blocked for us, but there was a program installed on all the computers that would display a message , "Eyes on the board", and shut everything else off whenever the teacher needed our attention. It also could block all domains but the listed whenever the teacher wanted our work to be done. I think this method works the best 👍🏻
Now in secondary school, in my intro to engineering class, nothing is blocked. Not even game downloads are blocked. So the teacher is talking and showing power points on a poorly lit projector...and the kids next to me are analyzing last week's basketball game. The kids in front of me are blasting zombies' heads off in a game or watching YT gamers. I'm guilty of going to other websites to get irrelevant classwork done (but I pay some attention, I swear!) The teacher can't get kids to really focus, and it doesn't help that he's really lenient in the first place.
We just need to find a balance, perhaps depending on the individual classroom, because while blocking helps the kids who can't focus, it also restricts something really useful.