Unless Steam support can recover your account, there’s nothing Bungie can do.

Unfortunately since March 25th Bungie completely shut down unlink system, they're not even able to unlink accounts throught support making no exceptions. This case of yours is a perfect case about how the actual policy do prevent fraud and illicit activities but don't protect users from mistakes and reverse decision. I have too a Steam account that I don't use anymore and I've decided to delete. Before it, I took a look to every services I have connected it, and Bungie.net is the only one who don't let in any way an unlink option. They should somehow review and reconsider actual policy and implement a better system like cooldown (like 12 months) between a linking or unlinking, like most other services already do. This would keep trust from their users.