The PiOS 12 Bookworm has been released at the same time as the Raspberry Pi 5, but can you still use 11 Bullseye or even 10 Buster with the Raspberry Pi 5.
There where a few changes to the way PiOS works in PiOS 12 Bookworm compared to PiOS 11 Bullseye. One of the key changes is that the window environment changed from the old X11 to the more modern Wayland. This can also mean that some programs don't work properly any more.
One notable program is RealVNC which many Pi users have been using for Desktop sharing. As there is no Wayland version VNC is now moved to Tiger VNC for Bookworm. I have tried a few VNC Viewers on my android phone but as yet been unable to connect to the Pi. RealVNC on my phone worked fine.
The other significant change is the network is now managed by Network Manager rather than DHCPCD. For most users that isn't an issue but if you have a Hotspot setup or other config that uses DHCPCD which has been used since 2012, then your setup will no longer work.
I have had to rewrite all my Autohotspot Scripts for Bookworm AccessPopup script
So if you want to keep using Bullseye or even Buster and want to use a Raspberry Pi 5, can you?
The simple answer is No.
When you switch the Pi5 on with Bullseye on the SD you will be met with this message
Part of the message says if you add "os_check=0" to the /boot/config.txt this check will not be done.
Well if you do that then the OS just doesn't boot.
This is because the Pi5 needs different core Linux image which are only available in the Official PiOS Bookworm.
There are plenty of other Linux distros that work on the Pi and there may be an option to get it working on older operating System. From the standard options you will need to move your projects to Bookworm before you can use the Raspberry Pi 5.