I agree with Kad that I think physical programs should still be provided as an option even if a theater pivots to primarily digital. But we already have a case study in QR code programs in many theaters off Broadway (CSC, Signature, MCC, just to name a few). They usually hand out a little cardstock page with the QR code, but it also has information like basic cast list/song list etc, which I think is a decent balance so you can still look up who the performers are during the show if you want. And fwiw I've never seen people actively using their phones to look at a digital program during a show. If people respect phone policy I think they would continue to not use their phones during a show, and if people don't respect phone policy, I don't know that their behavior would change all that much if programs go digital. Granted, a lot more tourists attend Broadway than off-Broadway and I'm sure they're the more egregious offenders of having phones out during a show (so I'm curious if people have noticed this more with touring houses that do QR codes).
Regarding understudies, unless people are out looking for it, I'm not sure there's a good way of marketing which understudies are on in a given night without slips, short of projecting it over the stage like super titles. With the existing QR code, is there any indication whether there are any understudies on at all or would you have to check online to determine if the full cast is in or not? Since ushers sometimes hold up placards about turning your phones off, I wonder if they could have one saying "understudies on tonight, scan this code" or something. But like others have said, if any of the above the title names are out, I'm sure people would already be notified in advance.