I'm not particularly old yet, in my mid-thirties, but I reacted like someone much older when I learned kids are allowed to carry around their phones all day at school.
Back in my day (when we walked to school uphill both ways), we weren't allowed to carry around basic flip phones. They had to be in our locker and only used before or after school.
When and why did it become acceptable for much more distracting and stimulating devices to be allowed in class?
I think it is precisely because they are more distracting. When the most addictive thing in phones was the snake game, kids did not bother to insist in using their phones all the time. Now, when you try to tell a pupil to put the phone away, it often results in a huge arguments, so eventually teachers gave up.
Never. There has never been a time when it was OK to use a phone in class. What happened is A) Some kids do take their phone out and play with them and either get caught or not B) Something happens and kids record it aka school fight videos. C) giant moral panic that has very little basis in reality.
It became "acceptable" because the teachers and admin were already on their phones constantly. I went to grade school from 2005-2017, when iPhones came around the adults got them years before kids did, I had numerous teachers that would sit on their phones half the class.
When parents themselves also became addicted and decided it was easier to give their kids phones than to parent them.