Depends on where you put the line between "open phone" and "baseband blackbox". Drivers are not an issue for phones like Librem 5 or PinePhone since they're using a separate modem module connected to the main SoC via USB and communicating over AT and QMI interfaces to which there are perfectly open drivers. The modem itself remains a vulnerable proprietary blackbox, but it does not have any access to your OS and you can cut it out from power while keeping the rest of the phone intact.

Open basebands are not something we're anywhere close to having though, for many reasons.