Hmm, I think this is so that, say, /usr/obj/usr/src/amd64.amd64 stays around for clean world but all it's subdirs are deleted. Not sure if that behavior change might break some folks (e.g. I sometimes symlink /usr/obj to somewhere else rather than using MAKEOBJDIRPREFIX as that is less cumbersome, and if someone symlinks OBJTOP then this will now delete the symlink rather than what it points to).

If you wanted to avoid tangling with that bit of POLA you could just make cleankernel a separate target and perhaps make it honor KERNCONFS to delete multiple kernel configs while at it, something like:

cleankernel: .PHONY
.for kernel in ${KERNCONFS}
KERNEL_CANONICALOBJDIR:=${KRNLOBJDIR}/${kernel}
.if !empty(KERNEL_CANONICALOBJDIR) && exists(${KERNEL_CANONICALOBJDIR}) && \
   ${.CURDIR:tA} != ${KERNEL_CANONICALOBJDIR}
     -rm -rf ${KERNEL_CANONICALOBJDIR}
     -chflags ...
     -rm ...
.endif
.endfor

The whole reason I took the * out is that it produced a arg list to the shell that was too long. At the very least this warrants a comment explaining why...
And that's a really obscure (and IMHO stupid) M/O wrt OBJTOP... If you do weird stuff, you'll get weird behavior....

And KERNEL_CANONICALOBJDIR can never be empty... it will always have at least / in it... :)

About half of the following should move to build(7), but I don't have the energy to do that today.

I resorted to a shorter way to do the same thing.

Out of paranoia this should perhaps be cd ${BW_CANONICALOBJDIR} && rm -rf *? cd might fail if someone's done something weird and removed permissions from the directory.

Both should be. Yes.