The DataAccessServices and WikibaseServices interfaces, and their sole implementing class MultipleEntitySourceServices, are left over from an older style of Wikibase code. During the Wikibase Service Migration, we agreed that they should eventually be removed (T276931, T277731) and made a lot of progress towards that goal (T276531, T276532, T276537, T276540, T276542, T276543, T276544, T277731, T277856, T278247, T280643), but didn’t fully reach it. As especially the WikibaseServices interface is a source of confusion due to its generic name, we should fully remove it.