As cofounder of raspbian I would like to clarify the situation. There is no takeover here just some clearing up of terminology.
The raspbian project still exists and will continue to exist to provide an armv6 hardfloat rebuild of Debian. We are not controlled by the raspberry pi foundation/trading, though they do support us by providing some hosting services and paying for some of my time.
On top of this base various parties including but not limited to the raspberry pi foundation/trading built customized images. In practice the majority of raspbian users used the raspberry pi foundation's raspbian images and their images were often just reffered to as "raspbian". Over the years the delta between "plain raspbian" and "raspberry pi foundation raspbian images" has grown.
For a while now raspberry pi have been considering 64-bit images, Eben sent me an email asking my opinion about nomanclature and I expressed that I would not be pleased about the use of the name "Raspbian" for images that did not contain anything from Raspbian. Using the term "Debian" would also be problematic as Debian are also building their own images for the Raspberry pi.
So Raspberry pi have decided to use the term "Raspberry pi OS" for their operating system images going forward. Roughly speaking that leaves us with the following.
"raspberry pi 32-bit OS" = raspbian base userland + raspberry pi kernels (32-bit and 64-bit) + raspberry pi UI and graphics stack enhancements + raspberry pi additional software
"raspberry pi 64-bit OS" = debian arm64 base userland + raspberry pi 64-bit kernel + raspberry pi UI and graphics stack enhancements + raspberry pi additional software