A newish and very welcome trend.
This sign from the Bob Dylan concert in Vancouver, 25 July 2017.
Whether being used for taking photos or videos, or used to relay pictures or thoughts via social media, cellphones can cripple live performances. They distract the user, those around the user, and the performers. Bono of U2 has referred to how mobile devices interfere with the ‘shared experience’ that a crowd has of being at a concert, and we think he has an excellent point.