>
> Can't we instead have a database of these messages, like we do with
> the "password" prompts?
>

I am not familiar with all grep options but I saw that:

‘-I’
     Process a binary file as if it did not contain matching data; this
     is equivalent to the ‘--binary-files=without-match’ option.

Just tested

  (setq xref-search-program-alist '((grep . "xargs -0 grep <C> --null -snHE
-I -e <R>"))

and it works fine on my side.
-- 
Matthias