Even if every scrap of WWII bombing records had been kept, preserved, and properly indexed for that - WWII bomber navigation was nothing near accurate enough for it to be very useful. I'd figure the builders just go with their local knowledge - of how many WWII bombs have turned up, and where, and things to watch out for.

BTW, Europe has similar problems with dud artillery shells, old minefields, etc. And it's not just WWII, nor just Germany. The most infamous is probably https://en.wikipedia.org/wiki/Red_Zone_(World_War_I)