Not strictly a dumbphone, but KaiOS phones (https://www.kaiostech.com/ https://www.kaiostech.com/) are sort of smart-ish. They're forked from FxOS, are easily

Not strictly a dumbphone, but KaiOS phones (https://www.kaiostech.com/ https://www.kaiostech.com/) are sort of smart-ish. They're forked from FxOS, are easily hackable, are compatible with modern telephony standards (so the death of 2G won't kill them), and come from multiple suppliers. They hit the "just enough" functionality for me: phone calls, access to smart-ish messaging, a music player, and the ability to act as a hotspot for a more dedicated device. I haven't got one myself (there's a particular model I was waiting on 6 months ago when I last sniffed around), but I intend to this year.