A summary and an update, now that Moode Audio 8.3.4 and 8.3.5 are out.
Both 8.3.4 and 8.3.5 are still based on kernel 6.1.21, which has the broken driver for the Boss 2. It appears from the
relnotes that this kernel was introduced in 8.3.1. (Personally, I can confirm only that the driver works in 8.2.x and does not in 8.3.3 and 8.3.5.)
Regarding updates and patches with the new versions: I have 4 RPi4+Boss2 machines. 3 were at at 8.3.3 and patched as described above, which results in a kernel version reported as "6.1.34-v8+". I successfully updated these machines in place from 8.3.3 to 8.3.5. The update log warns that the step which replaces the kernel fails, because the existing kernel version is newer. This also means that the patch does not need to be re-applied, you can just update and reboot as expected, and the working patched kernel remains.
My final machine was running 8.2.x (unpatched, because 8.2.x runs a working v5.x kernel). I forgot to note which revision of 8.2 and kernel version I was running. I successfully updated this machine in place to 8.3.5 and *then* applied the driver patch. It works fine now with a patched 8.3.5.
As mentioned in the final 2 posts by user pelwell on
this thread, the kernel patch will be in the next RPi kernel released after 6/20/2023 (with superficial research, I have not found a release date). The first Moode Audio version released after the fixed RPi kernel will have the fix. Meanwhile, patching the broken Moode 6.1.21 kernel is easy, and you need to do it only once: updating in-place to higher Moode point releases will not* overwrite your patched kernel with a broken kernel, but will overwrite your patched kernel after the upstream kernel is also patched. It's not relevant to Moode users, but potentially of interest: the patch is queued for Linux kernel 6.4 (upstream of RPi kernel), and it will be backported to earlier 6.x Linux kernels.
* ...unless a new Moode version bumps its kernel to >= 6.1.35 before the next (i.e. fixed) RPi kernel is released. I'm not sure if this could / would happen, but if it does, I'll revise / reply to this post.