Along with the recent launch of the new 8 GB variant of the Raspberry Pi 4, the Raspberry Pi foundation also decided to give its customized Linux variant a new name. Raspbian is now known as Raspberry Pi OS going forward and brings with it a new 64-bit beta variant designed to take full advantage of the RAM upgrade.
https://www.notebookcheck.net/Raspbian-is-now-Raspberry-Pi-OS-new-64-bit-beta-now-available.467162.0.html