I had researched dumb phones recently only to learn that the two most important functions of a smartphone (for me) is the GPS and having a decent camera, and these things are either crippled or missing entirely from dumb phones. It's simply easier to dumb down your existing smartphone, especially if you have an Android phone. In my case, I have a 2017 Pixel 2 XL running LineageOS, with a text only home screen launche…
That claim is a bit nuanced, IMO. Before I finally chose to move to iPhones, I tried my best to remove distractions from my Android phone, and I failed. iOS is less customisable, but comes with a configuration/ecosystem baseline that's far easier to tolerate, and insofar as it is flexible, the changes are far less painful than bringing an Android to that level.
Again, that's just my experience. YMMV.