Things are afoot in the world of Raspberry Pi operating systems. Outside of the usual OS updates this is usually a steady, predictable world, but not at the moment. The first big change is more of a rebranding of the Raspbian that we all know and love: Raspbian will be no more and in its place will be Raspberry Pi OS. It’s a case of being the same, but different.
Raspbian lives on - just by another name.
It’s not an entirely unexpected change. The Raspberry Pi Foundation is developing into an more global and professional organisation (was it anything but professional? - Ed) and so it makes sense that its core operating system is “branded” with the same name as the hardware that it runs on.