It seems pretty optional in the US at least. My phone has been broken for extended periods of time before. But different story trying to use budget European airlines like Wizz that require an app to get a boarding pass.

It's also very much optional in Russia as well. Everywhere still takes cash, everything can be done on paper. And speaking of air travel, most airports actually require a printed boarding pass. IIRC you can use an electronic boarding pass in SVO since relatively recently, but I've never done that myself.