I’m a little late in my reply but I believe they stopped SMS support because it’s pretty expensive in the long run. Signal took off over lockdowns like a lot of platforms, and SMS texts cost quite a bit (at scale) to route and process. My anecdotal evidence (take it or leave it) is that I worked at a fairly major ecomm tech company around that same time who discontinued 2FA verification via SMS in regions like India (etc) because collectively it was costing the company millions to use that route for that purpose.
They actually offloaded the 2FA flows to free (for the user anyhow) services such as Google Authenticator and Authy etc etc (which those companies now have to spend the money on each SMS interaction and/or server costs, not the one I was working for.
Ultimately I think Signal did it because it was costing them a lot of money with not a lot of benefit as more people adopted the platform.