I like to keep on top of the dumbphone beat–phones that don't hog your attention or provide much of a habit loop–and things have moved on a lot in the last year or two. One one hand, the last 2G carrier is sunsetting its network, meaning that many existing "popular" dumbphones will stop working. On the other hand, there's now a solid range of modern options from super-cheap to veblenesque. Jose Briones, author of Low Tech Life: A Guide to Mindful Digital Minimalism, posted a roundup of the current field on YouTube and as a lengthy posting on the dumphones subreddit. The TCL Flip 2 is so inexpensive that you can buy a crate of them for the price of a premium smartphone.
At $20, this phone is amazing for those looking to get a basic experience with some extras. The phone is customizable and can install apps (like WhatsApp, Signa, Spotify, etc.) if you follow this guide (https://github.com/neutronscott/flip2/wiki). I love the price of the phone and if you find one unlocked, it will work with most major carriers in the US (and maybe beyond).
That said, I'm really here to recommend something in particular I impulse-bought and have been fooling around with: the Cat S22 flip phone [Amazon]. It's just been discounted to $67, unlocked, and is a good "transitional" device that's too dumphoney to sit there staring into but smartphoney enough to have functional apps when you need them.
