Some time ago (within the last couple of years), the Shifter Undead and Construct shapes were changed because of this very reason. Regrettably, this change has not been fully documented in LORE, but announcement of such is given in the update notes for version 3.31.
Druids, and by extension Shifters, are not completely prohibited from using metal. Some of their allowed weapons are metal (dagger, scimitar). Though these are not the weapons visible/available when shifting to another form, at least not by appearance, it can be said that either these shifted "weapons" are formed out of what the Shifter is already carrying or that they only appear to be metal and bear similar characteristics.
Where the Undead and Construct shapes were firmly against the Druid ethos, and thusly changed, we didn't have a significant issue with the other forms.