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I see this problem a lot, usually when the old tub had a mesh and mortar build up then tile.
You can use a waterproof drywall as a "filler" then add your backer board over the top, (you should be using a concrete based backer board to lay your tile on)
BUT a much better option is to get your self some pine strips of wood from Lowe's/Home Depot and nail these to the studs, then screw on your backer board, basically your extending your studs out to fill the gap. Also while you have to wall open it's a great idea to install some R13 insulation in the wall cavities, even if these wall are not exterior it helps cut down bathroom noise transition to adjacent rooms.
On a separate note is that galvanized pipe I see connected to the copper ?
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