I have a powered-up dumbphone, Sony Ericsson w580i. I have recently used it as a flashlight given the lack of power locally.
But it isn't the phone which has my SIM card in it, that's a (comparatively) modern iPhone 6S. And prior to that (and a slightly newer, but less spacious, iPhone SE), I was on the Burning Platform, Symbian, from 2014 - 2019.
I do not have e-mail set up on my iPhone (intentionally), nor do I post to or browse CivFanatics from it. Those are strictly, and intentionally, laptop/desktop activities. It's part of how I demarcate the "real world" from the Internet-connected world. That said, there are times where it's useful to be able to look up the hours of a restaurant or the address of a location in Firefox or HERE Maps.
I think the big weakness of a dumbphone these days would be the general lack of support for group texts. Add that, and a halfway recent version of Opera Mini, and I could probably get by with a dumbphone today, much like I got by with the Burning Platform well past its best-by date. But that is a hard ask to go back from. I remember when one of my colleagues had his modern smartphone break, and he went back to a middling (not the latest model) BlackBerry, group chats was what he missed the most.
But aside from that? Yeah, I'll keep using my iPod for Pandora and mp3s, and I like my GPS apps for outdoors stuff. But take that out, and if you gave me a dumbphone with group chat and Opera Mini, I'd probably get by just fine.