Never.

Cities are still razed in war. ISIL is busy doing it right now in Syria and Iraq.

WWII saw many cities razed, including two by atomic bombs.

What exactly do you mean by "razing"? The media trope of completely sacking a city and rendering it virtually uninhabited (or uninhabitable) is somewhat rarer in reality than it is in folk-memory or modern fiction. Small settlements of less than a few hundreds of people get wiped out all the time, but actually thoroughly sacking a city, murdering or enslaving most of the inhabitants (combatants or otherwise) and then leveling or burning the rest, was fairly rare -- which is why it's nearly always depicted as a divine or diabolical atrocity in contemporary narratives. There's lots of military operations or even outright war crimes and genocides that have killed hundreds of thousands of people at a time, even in the last century. But actually razing a city to the ground? World War II at that latest, maybe even longer ago that than depending on semantics. Certainly lots of cities in Germany, Poland, and the USSR (as well as Japan) saw more than 50% of their structures reduced to entropy-rich rubble before the end of the war.