Recent commits suggest Signal is preparing a paid subscription for backups.
Recent commits suggest Signal is preparing a paid subscription for backups.

Signal-Android/app/src/main/res/values/strings.xml at 7ee1b1386b6a6d5bf9ff90d6b6e20ec466692b7a · signalapp/Signal-Android

I imagine this would be backing up to Signal's servers, not restricting local backups? Seems like a harmless way to raise money; I'd wait for a blog post for them to explain their reasoning.
Yeah I hate the overt monetization of absolutely everything in general, but charging for optional persistence to their cloud seems pretty reasonable.
I can confirm that, as someone else who has been passively listening to discussions about it, that's the exact impression I got.
Information is available on the unofficial Signal wiki, if you feel like digging around.
Edit: apparently discussions go back at least a year. Here's one titled "Signal is getting Cloud Backups"