Exactly. I would want:
-Phone/texting -Messengers like WhatsApp/Telegram/etc. -Maps -Music/podcasts
Ideally with e-ink.
I want something that doesn't compete with a full-fledged smartphone (which I'd still keep for travel and stuff). I want something to take to the office, hanging out with friends, etc, where bringing the smartphone becomes a decision.
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.