That's news for me. I thought they are silently ignored. Do you have an
example of such a warning?

Not really. It was just assumption. It seems you are right, they are just
ignored silently. But item 2 is still relevant and it was actually confused
me when I tried to figure out MTD configuration for my platform.
Anyway, garbage should be taken out from time to time.

Seems like the kind of change that can only be reviewed by a script.

Good point. Because these patches were done by script to start with.
Anyway, I reviewed result patches manually to be completely sure.
You can write verifying script though, if you need to, it wouldn't take
more than 2 minutes (it takes one-liner, actually).

I seem to remember Greg stating that defconfig files are on their way
out. Did I remember that correctly?

Maybe they are. But they still in place, and why it's so, we should
maintain them up-to-date. Ideally, this kind of work should be done by
someone who introduces changes into Kconfig files. So I just fixed
that oversight.