I don't think anyone has reviewed org--extended-face yet (I first
drafted it in bug#42184#17), so don't hesitate to call me out on it if
something looks off.  FWIW I saw no warnings when running "make" with
Emacs 26.3 nor with 28.

I made patches for both maint and master because

- the patch for maint has a small conflict in org-compat when applied
  against master,

- I'm not sure which branch will eventually be synchronized with
  emacs-27 (assuming we'll include this fix in 27.2).

Thank you for following up on this.