Journalists (as here) don't usually get to choose the communication software their sources are comfortable communicating over. They install whatever's required to get the story. And they likely wouldn't install an OS that doesn't let them install such apps.

That's a niche use case. You might not be able to choose what app they require to communicate over, but you can choose what device to install it onto, like a burner, couldn't you? Some apps you might not mind on your personal phone, others you probably do.

Well, yes, but if you think about it, the whole point of a journalist's work phone is just to aggregate a bunch of "burner" accounts. And that's exactly what an attacker would want to steal from a journalist: conversations between them and (or contact details of) another source.

Which is all to say, ideally a journalist would have N phones, one per source. But that's impractical.