Eli Zaretskii <eliz@gnu.org> writes:

>> (defcustom org-num-skip-tags nil
> ...
>>   :safe (lambda (val) (and (listp val) (cl-every #'stringp val))))
>> 
>> Note that org-num.el where `org-num-skip-tags' is defined does have
>> (require 'cl-lib).
>> 
>> IMHO, there is some problem either with loaddefs generation or with the
>> fact that loaddefs file is compiled at all.
>
> No, the problem is that this variable uses cl-lib stuff for safep
> functions.  IMO, this is not a good idea, and the above situation is
> precisely the evidence for that.
>
>> Of course, we can always convert that lambda into a proper function,
>> which will likely get rid of the warning.
>
> Yes, please.  Emacs 28 and later has list-of-strings-p, but if you
> want to support older versions of Emacs, perhaps that's not usable.
> Or maybe we could add that to compat.el.

Org cannot directly use compat.el for now.
I am attaching tentative patch that defines a compatibility alias and
autoloads it.
Is this approach good enough?