Andrey Orst <andreyorst@gmail.com> writes:

> (defface foo...)
> (when (>= emacs-major-version 27)
>   (set-face-attribute foo... :extend t))

Unless I'm mistaken, this has the disadvantage of making Custom confused
as to who changed the face, i.e. M-x customize-face RET foo... RET will
say that the face was "CHANGED outside Customize".  For all intents and
purposes a package author may wish the face to be described as
"STANDARD" as long as no theming or user customization has been applied.

Maybe some backquote trickery may help?