For Android users, take a look at BaldPhone. Fully Open Source and primarily aimed at seniors, does a pretty good job at simplifying the interface making most important actions easier to perform. It does not turn a smartphone into a dumbphone (I don't think that is even possible without removing entire parts of the system, possibly bricking the device) but the simpler and immediate interface makes it a lot more usable.

https://github.com/UriahShaulMandel/BaldPhone