Just ordered a Nokia 8110 to try out. It runs KaiOS (a fork of a fork of the defunct FirefoxOS – based on linux), and WhatsApp support has finally arrived for it. It seems like a good compromise between a basic phone for calls and a smartphone, while being relatively cheap with Amazon UK having them on offer at the moment (and of course now the ability to use WhatsApp on a 4G/wifi network).
It can also be rooted, so it would be possible to add/remove specific features (such as SMS texting, the web browser or restricting the ability to install WhatsApp) as well as hard-setting CleanBrowsing DNS for family-friendly web filtering on the small browser.
Shame it doesn’t have a qwerty keyboard though. Any other feature phone users here?