Someone could wrap a phone in tinfoil pretty quickly or have one ready-made in the shape of a pouch. Or start carrying a special pouch that stops signals.[0]
Either way once you remove the phone from sight, the suspicion is probably over. The only next step is to kill the signal.
Seems undesirable if it doesn't automatically silence the phone by disabling incoming calls. If they still ring while in a sealed back, then I don't see the "artists" they're marketing yondor to, being very happy.
If I'm someone who works in actual life and death related matters and on a regular on-call schedule, I need to be able to
1) Still continue to have a life
2) Not have my phone completely disabled just because I went to see a comedy gig.
Having to leave the comedy gig when my phone starts vibrating in the bag is a good compromise, as is wearing a smart watch that can notify me exactly what the call or message is about before I do.
The selling point is to reduce visual / audible recordings of the performance, allowing artists to say or do things that may be problematic when taken out of context.
Either way once you remove the phone from sight, the suspicion is probably over. The only next step is to kill the signal.
[0] https://www.overyondr.com/howitworks