I think if you ask 5 people what dumb (enough) phone meant you'd get seven answers.

For some anything after SMS is not dumb enough (ok, this still exists), others would be ok with just a browser, and you said Maps and WA/etc/.

I am not even sure I could give you a good enough definition, I'm kinda happy enough with Android and just ignore any dumb/minimal angle, because if I only take a phone with me I generally want to potentially use it as a computer replacement. There are really only 2 people I call regularly.