Still, I think an open source & hardware feature phone has been in the real of possibility for quite a while already so it's surprising such project has not showed up earlier.

An open device possibly with some room for customization by custom lightweight software could be very welcome for people who carry current aging or low-quality feature phones, either because they don't want a smartphone or have the feature phone as another isolated device just for calling/SMS.

(I think some software customization, say by enabling/disabling modules at firmware build time, similar to for example Linux kernel build, should still be possible. This avoids the "feature phone curse" where due to the fixed phone software it is often hard to find the one feature phone in existence that happens to match what you actually want it to do.)

Also an open easily customizable device in the feature phone class device could find use in various projects where you need a lot of custom end user terminals or even simple SMS/data modems for machines.