This is the first time that somebody from the Raspberry Foundation is telling everyone not to upgrade to the New OS: Raspberry OS - Bullseye.
 
Though there are many improvements to the OS, is it discovered that drivers are missing apps broken due to System Links being broken, etc., etc. As is, Bullseye was released without a proper testing.
 
The 32Bit version (for older R-Pies) is more intact than the 64Bit version (for the R-Pies 3B+, 4B, 400, and Computer Module 4). It is with many issues.
 
This is similar what is happening to Ubuntu on their latest release, as it too is broken in many ways and the Ubuntu Forums is in flames from the issues with the OS. Both Raspbian and Ubuntu are from Debian Linux Kernel, and most of the problems are stemming from Kernel issues.
 
It is not recommended for anyone to download Raspberry OD - Bullseye until further notice though the website still has it as a downloadable link.
 
The video explains it in detail and in the end show how to access the Raspberry Foundation's OS Archive to download the Older OS for the Raspberry Pies. This video is a MUST SEE!
https://www.youtube.com/watch?v=QcRmfoZH6h4