Eric Sunshine [off-list ref] writes:

(2) In practice, I found that even after coloring those annotations in
red, it was still easy for the eye to glide right over them in the
output without really noticing them. Switching it to bold red helped a
bit, but my eye still glided over them sometimes. One possible reason
that the eye was able to glide over them may be because the "?!FOO?!"
annotations are very short bits of text buried in the much larger and
textually noisy test body.

Maybe partly because I work with black-ink-on-white-paper terminal
setting, and maybe partly because my color perception is suboptimal,
I learned to use "[diff.color] old = red reverse", because non-bold
red letters do not stand out enough.  Perhaps you may want to try
reverse output to see how well it makes them stand out for you.

I do not think if configurability like "git diff" has is necessary;
it would be overkill.  I personally do not mind more noise "?!"
around the keyword, especially since these are only shown when there
are problems detected.