What Hunger Games did they introduce the cannons to signal a tributes death?

We're not sure when it was introduced, but it's likely they were added sometime between the 10th and 50th Games as the Capitol gradually turned the Hunger Games into the highly produced televised spectacle seen in the original trilogy.

The books never give an exact Hunger Games number for when the cannons were first introduced to announce a tribute’s death. We only know that the practice was already established by the time of Katniss’s Games.

It was likely added as the Games evolved into a more organized televised spectacle, sometime after the earlier Games and before the events of the original trilogy. So there isn’t a confirmed canon answer such as “the 25th” or “the 50th Hunger Games.