> Are you saying that _all_ the faces will have to be modified to make
> them extended? IOW, are you saying that this feature is wrong with
> most or all of the faces?
I don't know about /all/ faces, but I have experienced a lot of visual changes
when using `doom-one' theme provided by `doom-themes' package paired
with at least these packages: magit, ediff, solaire-mode, org-mode.
> The assumption behind this feature was that the absolute majority of
> faces don't need to be extended. If you say this is wrong, can you
> show enough examples to back up that?
I understand this, and maybe package maintainers should adopt the change
but since Emacs doesn't ignore unknown attributes, this may result in a lot of
extra code in order to support both pre-27 Emacs, and 27+ Emacs to make
different versions look consistently.