Solved, thanks! :-)

;;;_ borrar todas las etiquetas al cambiar el estado a HECHO
(defun borrar-etiquetas-tareas-hechas (plist)
   "Borra todas las etiquetas al cambiar una tarea a =HECHO=."
   (when (eq (plist-get plist :type) 'todo-state-change)
     (when (string= (plist-get plist :to) "HECHO")
       (org-set-tags-to nil))))

(add-hook 'org-trigger-hook 'borrar-etiquetas-tareas-hechas)

El 27/06/2023 a las 14:05, Ihor Radchenko escribió:
> Ypo<ypuntot@gmail.com>  writes:
>
>> I have tried gptel inside emacs (I'm sorry, but I need to tell it) and I
>> got this code:
>>
>> #+begin_src emacs-lisp
>> (defun my-org-trigger-function (state)
>>     "Remove all tags when state changes to 'DONE'"
>>     (when (string= state "DONE")
>> ...
>> (add-hook 'org-trigger-hook 'my-org-trigger-function)
>> #+end_src
>>
>> Surprisingly 8-) it doesn't work. Any help?
> org-trigger-hook is a variable defined in org.el.
>
> Documentation
> Hook for functions that are triggered by a state change.
>
> Each function gets as its single argument a property list with at
> least the following elements:
>
>   (:type type-of-change :position pos-at-entry-start
>    :from old-state :to new-state)
>
> Depending on the type, more properties may be present.
>