Here's a detailed description of ambassador Neville Henderson being received by the foreign ministry in Berlin in 1939. Perhaps the most famous example.
He came in looking very serious, shook hands, but declined my invitation to be seated, remaining solemnly standing in the middle of the room.
'I regret that on the instructions of my Government I have to hand you an ultimatum for the German Government,' he said with deep emotion
This site has a description of Ciano handing out Italy's declarations of war on June 10th 1940:
On 10 June, dressed as a major in the Regia Aeronautica, he handed the Allied ambassadors Italy's declaration of war
When did this formal (slightly awkward) practice of making an official "declaration of war" via an ambassador or foreign minister cease?
It seems as if it stopped in 1945 but am I right? Thinking of the major conflicts initiated by individual sovereign states since WW2, such as Iran-Iraq in the 1980s, China & India, India & Pakistan, the Six Day War, the Falklands, in those examples I am nearly certain there were no formal declarations of war of the kind Neville Henderson or Galeazzo Ciano delivered.