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.
They do not.
https://www.wizzair.com/en-gb/help-centre/check-in-and-board...
https://help.ryanair.com/hc/en-ie/articles/39758330098577-Wh...
Will see about Wizz, maybe it was only Ryanair.