By working with the KDE Store (https://store.kde.org/browse/cat/560/order/latest/), I realized that the store elements are assigned to a single user and cannot be changed by others. Normally I think this is a security feature, but for projects with more than one maintainer or where the maintainer has changed, it is a big problem not being able to change or remove obsolete items in a certain category. See https://store.kde.org/p/1201735/ for an example. The skrooge maintainer told me that he cannot remove this obsolete item, which causes people to report invalid bug reports related to this item and reduces the quality and usability of the product that uses it.
It should be possible to assign additional users to a category, who should be able to change or delete items contained in it.