Hey Lars,
Just like with Debian, on Raspbian the older releases are moved to the archive site, where they remain available (and installable). Raspbian archived sources are available at raspbian archive and the SD card images at raspbian/images (from 2012’s Raspbian „Wheezy" on 
). On Debian archive, they even have the first Debian that I used )
That’s what I meant with the bbserial kernel module being “hardwired to an ancient kernel release”. It’s a dirty hack that was never replaced by a serious solution by the buildbotics main developer. Further integration of the kernel module into the OS was dropped as soon the serial interface to the AVR Mainboard that it provides did work. You know that: Once you get something to work, you stop working on it. Problem is, with the present situation of being “hardwired” to an ancient kernel source, you can’t compile this module if you updated the system’s kernel to a recent version, and if you don’t have this kernel module working and running, you have no CNC controller, but just a Raspberry Pi, with a CNC User interface that does not find the AVR mainboard with the CNC driver hardware.
On Debian/Raspbian (and its derivatives like Ubuntu), the only serious solution for a separate kernel module like the bbserial module is to integrate it into Dynamic Kernel Module Support (DKMS) so that the module is recreated automatically after every kernel package update on the system. And it would be necessary to make ‘bbserial’ and ‘onefinity-firmware’ or ‘bbctrl-firmware’ each a proper Deb package (which is still not the case 
) that have their package dependencies defined, and be installed, deinstalled, upgraded or even downgraded with apt. How one can learn to do this is explained in debian.org/doc/ → Introduction into Debian Packaging and of course debian.org/doc/ → Debian Policy #Binary packages has to be observed.
Note that when you use Raspberry Pi OS (formerly “Raspbian”) on a Raspberry Pi hardware, instead of some other Debian-derived OS, you benefit of the proprietary packages raspberrypi-kernel, raspberrypi-bootloader, and raspberrypi-sys-mods, that are not part of free and open source Raspberry Pi OS, but that get fetched from the repository defined in /etc/apt/sources.list.d/raspi.list. Those packages are not fetched from Raspberry Pi OS repositories (like http://mirrordirector.raspbian.org/raspbian/ and http://archive.raspbian.org/raspbian/ as defined in /etc/apt/sources.list, but from the repository of the Raspberry Pi Foundation at http://archive.raspberrypi.org/debian/.
raspbian.org is not raspberrypi.org!
I got one a few monthes ago but it was the last one available at that time. But why not use Raspberry Pi 4. Since it has much more graphics capability, it could show the camotics 3D toolpath simulation which the Pi 3 can’t do (provided the mesa 3d acceleration graphics drivers get installed)
Image: In order to see the 3D toolpath simulation, it is necessary to use the User Interface from a remote computer that has more graphics capabilities (and some 3D graphics drivers installed). This simulation is not shown on a HDMI monitor directly connected to the Buildbotics/Onefinity CNC Controller.
See also Why the Raspberry Pi <= 3 is slow.