Aside from the Rotary Un-Phone, there are pretty much no dumb phones anymore. Those that market themselves as dumb are just reskinned full-fat platforms.

Even almost all flip phones are smart phones with a dumb skin, as they run either Android or KaiOS.

The main reason why I would switch is for device security - a true dumb phone OS that operates purely out of the ROM and has no ability to install anything that could survive a reboot.

And for something that primitive, it would be a flip phone on par with the Motorola StarTac. Simple black-on-green screen, low-res display, with a calendar and address book as the only non-phone functionality.