But what is the “real” difference between destinations on the lanes or on the way after it ?
Since one can also see which lanes are connected to which ways after the split, any navigation app can put you in the right 2 lanes and announce “B” whether B is mapped as destination on the way after the split or as in your German wiki example or as A|B|B

The benefit of mapping it on the way after the split is that you do not have to repeat the destations, the thing you want to avoid in the first place. There are plenty of motorways with more than 2 lanes where you have the repeat the same destinations 3 or more times. The value of the destination:lanes tag can become too long for OSM values or pretty hard to maintain / understand.