If playing a priest deck, if the opponent silences your Elwynn boars before they die, they still seem to count towards the condition for getting the 15/3 weapon. How does this make sense? Also, the priest spell that resurrects deathrattle minons also raises minions who were silenced and then killed. This also doesn’t appear to make sense; it seems like an arbitrary decision that goes against what you’d expect based on the rest of the rules of the game.
Elwynn boars deathrattle states that you will get the Sword of a Thousand Truths if you had 7 boars die that game. So if you silence them and kill them, they still die and correctly increase the number of boars that died.
If you silence the last boar after 7 boars died and kill it, then yes - Sword condition will not trigger.