Let me start by saying that I find phone use in theatre really distracting, and it makes me angry when I see it happening.
But that's because I've been conditioned to see that as someone breaking the societal standard we have set in place: to sit quietly and still in a dark theatre and watch the stage attentively. And that standard was set by a more elitist - traditionally white - culture in the arts. That isn't to say that it's necessarily a bad thing, but we have to acknowledge that it's not every culture's approach to theatre-going. And how do we reconcile those differences without alienating Black audiences, and audiences of other cultures that don't subscribe to the traditional, elitist norms of how to treat theatre as "sacred"?
To me, the answer is fairly simple (though not perfect). We need to create space for other kinds of theatre experiences, and establish those expectations through communication. And this is already starting to happen. For example, in the past few Doninique Morisseau plays I've seen, she has written a note for a program insert that gives audiences "permission" to respond vocally to play. Another example: at the beginning of Summer, Diva Donna tells the audience that if they want to sing or dance along, they should. In certain specific theatrical experiences like Privacy at the Public, or Ivo Van Hove's Roman Tragedies, they have encouraged phone use as part of the show experience. And when these things are communicated to the audience, these audience behavior can no longer be "distracting" from the show experience, because the artists have deliberately allowed them to become PART of the show experience. Some else above even pointed out that back in Elizabethan times, rowdy, responsive audiences were the norm, and many of Shakespeare's plays have been written specifically with the expectation that the audience would be responding.
And if we make theatre more accessible, and break down class and cultural divisions in the arts, we can see improvements on both ends. Audience members who aren't used to the expectation of sitting silently can adjust more easily to that environment if that's what the playwright wants for their work. And audience members who can't focus on a show if people are using their phones or responding vocally can learn to relax and accept that as part of the communal experience of seeing theatre.
And I realize that phone use is different from being a vocal audience member. But I think it's all connected. Part of the reason phone use is so distracting is because our theatres are dark, but again, that norm is part of our socially-constructed perception of "elite theatre-going."