Are you sure it's a branch? I looked it up, and the Raspberry Pi OS web site says "Raspberry Pi OS (previously called Raspbian)". Makes it sound like it's a rename.. https://www.raspberrypi.com/software/ 
"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
https://forums.raspberrypi.com/viewtopic.php?f=66&t=275380&sid=1a468f226394ccdd f4654a3d3d90cb7d#p1668466
The Raspbian people also made a statement to Tom's Hardware awhile back...
https://www.tomshardware.com/news/raspberry-pi-os-no-longer-raspbian
"Raspbian is an independent open-source project, which maintains a rebuild of the Debian armhf port modified to run on armv6 hardware. Starting from an armhf bootstrap, NEON fastpaths and a small number of armv7-only scalar instructions are removed through build settings). Using Raspbian allows us to take advantage of the hardware floating point unit on the original BCM2835 AP, which using the "official" Debian armel port wouldn't. 
So when we say 'Raspbian image' what we mean is 'our Raspberry Pi operating system image built using the Raspbian repos', but what people hear is 'Raspbian is the name of Raspberry Pi's operating system.'
This confusion is fairly harmless, until you get to 64-bit. The 64-bit equivalent of 'Raspbian' is 'the Debian arm64 port.' It isn't 'Raspbian' or 'Raspbian64' or anything like that. So we're overdue for an actual name for the OS, and we're taking the opportunity to clear up the confusion around the 32-bit OS at the same time."
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