KDEInstallDirs6 has been set to use a subdir with the 6 postfix instead of 5 subdir. One would assume that the storage for user modified data would match the subdir

This raises the question though how to migrate existing custom user data from the KF5-based application. The format so far is compatible, that is not a reason to change the path.

Perhaps indeed should also KF6's KXmlGUi continue to use the 5 variant, denoting the format version?