That's what is nice about signals implementation is it stands. It supports acting as the SMS default app on android and defaults to signal when it can.
Sure they handle SMS, but the real problem here is that Signal is just another walled garden: they have an overtly negative stance towards alternative clients, while also having very bad support for anything besides android/ios: they have a bad desktop client and they don't have a nice library. Altogether this means that Signal is overtly and willingly against things like Pidgin / multi-protocol clients or overlay, which is what the users want (ie not caring about protocols).
Signal doesn't want to deal with SMS anymore, which from an engineering and high-stakes security pov is a completely valid decision. Yet if it had clean and open local API or a simple and portable client library, or had a stable server API, then someone else could provide multi-protocol clients, tailored to each platform in a secure and stable way.
Now to my actual question: How is Beeper compatible with the ToS of platforms like Instagram and Facebook that, to my knowledge, don't allow their users to use 3rd-party apps? Case in point: I recently wanted to use a FOSS 3rd-party messaging app for Instagram and my account got promptly banned.
Question 2: Do you support full message backups in a well-documented format?