This is a good idea and the reason she probably doesn't have it is that her model and splash haven't been updated since her lore update. When she gets some model updates, I would hope Riot would add the necklace in.
I think it'd be great for Riot to see this idea, but unfortunately, Gameplay is not the correct place to talk about Ahri's splash and in game model. The better place to post this thread would be in Story, Art, & Sound. I am going to lock this thread because it doesn't belong here, but I highly recommend you recreate it over there so we can get Riot to see and consider this idea.
If you have any questions about this lock, please come to the Discord.
Thanks
[sg-ahri-2]