I have an old Nokia N900 phone that runs a custom alpine linux-based rom (postmarketOS). Given the limited hardware resources, small screen size, and sliding keyboard, you'll be mostly running terminal applications on this phone (some GUI apps works but the experience is mixed). There are some unofficial terminal clients for Signal [0], and iirc hacker news works with the lynx terminal browser. The N900 might work for this scenario as a feature-minimal phone, but depends on your use cases.