What I get when I purge old kernels is...
/etc/kernel/prerm.d/dkms:
dkms: removing: (5.14.0-4-amd64) (x86_64)
Error! Arguments <module> and <module-version> are not specified.
Usage: remove <module>/<module-version> or
remove -m <module>/<module-version> or
remove -m <module> -v <module-version>
modules have to be removed manually specifying module and kernel version
I guess this started a couple of dkms releases ago, but at that time I
didn't have the time to report and later I forgot about the problem.
fyi: I'm force merging duplicate reports to #996104. The bug was fixed upstream in 2.8.8, but Debian upgraded to 3.0.3 in the meantime (bullseye never had this bug, because it only has version 2.8.4). I assume folks tracking testing are going to have to manually downgrade to 2.8.4, or upgrade to 3.0.3, because I think it's unlikely that there will be a 2.8.8 release just for testing.
Bug 1006431 may have been fixed at the same time as bug 996104, but this was not the same bug: bug 996104 was typos in the /etc/kernel/prerm.d/dkms script, so that the dkms remove -m $name -v $vers -k $inst_kern -a $arch command failed due to incorrect arguments. So what I did was to run the correct command manually. This had the effect to remove the installed module, but was still leaving some files there. Or perhaps this command is expected to be run while the kernel is still installed (it is normally run as a prerm.d script, while I ran it after the kernel was removed).