There are three big reasons why we’re removing SMS support for the Android app now: prioritizing security and privacy, ensuring people aren’t hit with unexpected messaging bills, and creating a clear and intelligible user experience for anyone sending messages on Signal.

To me, all of those reasons are BS and easily gotten around. “Unexpected messaging bills?” Have a popup that warns you that this user doesn’t have an account and is about to send a SMS, potentially incurring a cost, as an example.

They just didn’t want to maintain the code and chased some users away. https://www.signal.org/blog/sms-removal-android/