Now that Ability Patches appear to be confirmed as obtainable in the base game, I'm feeling a bit more confident that Egg Move transference is still kept as well. This is combined with how the feature was also present in BDSP, and the fact that there seem to be one or two Pokemon with "egg moves" that they cannot get through breeding at all (Tinkaton with Ice Hammer).
I'm going to continue leaning on my hunch that its held item related. Someone elsewhere suggested it could be Mirror Herb, which is a brand new item which may be more likely to have a new hidden secondary function. Unfortunate that its a feature that seems too tedious and time-consuming to test, so we may not know for sure if its still in until the game comes out.
EDIT: That's not even considering the possibility that its now tied to some new Move Tutor style NPC, given that there are also some unknown "tutor" moves in the datamine last I checked.