I don’t think ive heard about any privacy issues regarding modems. They convert your data into the Level 1 format so that it can be moved to the next hop. There isn’t really anything to spy on, and its very hardware dependent (hence no open source software that can standardize across each device). There might be open source modems out there, but your ISP probably doesn’t support them.
To be additionally fair, Android still has phones out there in use that still dont have the RCS feature, and never will because those phones are no longer supported.