Custom irons backordered?

Makes sense. Adding an inch really can throw the swing weight off, so they need to create a heavier head. Pretty rare so they probably don't keep a lot on hand for that fitting.

The heads due to manufacturing processes don’t all arrive at the same weight. Titleist separates them into 2-3 bins. They can always add weight to a club while building to increase swing weight. They don’t have a process to grind weight off.
Longer shafts increase swing weight with the same weight head since the fulcrum is longer. So the bin for the lighter heads must be low. Standard swing weight will be D2-D3.
If you were fitted with a longer shaft and you liked the feel, your fitter can check the swing weight of the fitting club. It may be D4-D5 with the longer shaft. Titleist may be able to fulfill the order at a higher swing weight. It is a specification that can be customized in the order.