I think Jaheira being non-romanceable is more to do with honouring the player choices of BG2. For some players, their MC partnered up with her and lived happily ever after. Some of those MC's were long-lived and in player head-canon are still alive.
I wouldn't be surprised if Jaheira in BG3 keeps her private life private to protect a loved one, or their memory of, and is therefore unavailable.
