Eli Zaretskii <eliz@gnu.org> writes:

#+begin_quote
How did you see that, and where?
#+end_quote

Mostly in third-party major-mode.  But also in built-in ones, for
example css-mode that have font-lock-keywords-case-fold-search set to 't.

I haven't seen any minor mode that uses this variable.  I guess the
reason is to avoid conflicts with the major mode.

#+begin_quote
We need to understand the issue better to reply to this question.  So
please provide more details.
#+end_quote

My idea is to add a kind of note to
font-lock-keywords-case-fold-search documentation in lispref tex file
to avoid any confusion that this can be used in minor modes.

Something like this:
#+begin_src diff
diff --git a/doc/lispref/modes.texi b/doc/lispref/modes.texi
index 768d78229aa..02e7a6f553f 100644
--- a/doc/lispref/modes.texi
+++ b/doc/lispref/modes.texi
@@ -3573,6 +3573,9 @@ Search-based Fontification
 @defvar font-lock-keywords-case-fold-search
 Non-@code{nil} means that regular expression matching for the sake of
 @code{font-lock-keywords} should be case-insensitive.
+
+@strong{Warning:} This variable is intended to be used by major modes
+and it's not recommended change it by minor modes
 @end defvar
 
 @node Customizing Keywords
#+end_src