I think "Grub Themes" is the only item missing at the present in System Settings "Appearance" category while there are so many available at https://store.kde.org/browse/cat/109/order/latest
One objection I can see is that it's also equally at home in the "Startup and Shutdown" category, but you could easily say the same thing of the Splash Screen KCM, which is in the "Appearance" category.
makes sense to me.
A possibly relevant merge request was started @ https://invent.kde.org/plasma/plymouth-kcm/-/merge_requests/8
Git commit 1b3f6a3cd2450038b867bfbcaa526f621c54ed1a by Nate Graham.
Committed on 19/05/2021 at 15:09.
Pushed by ngraham into branch 'master'.
Move to appearance category in System Settings
This KCM is 100% appearance-based, with no behavioral settings
whatsoever. This is generally the case for KCMs in the appearance
category, so let's put it there.
M +2 -2 src/kcm_plymouth.desktop