In the past I would have agreed. But recall that we were discussing this in another thread (Question on Event reply in webhook - #4 by Phil_Seeman) and it seems like fairly recently, webhooks started returning for subtasks of tasks in a monitored project. I never got an answer from Asana in that thread but in fairness to them, that was right around when Joe left. So I’m going to try it again here 
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What say you, @Ross_Grambo? Could you check and see if it’s true that webhooks now fire for subtasks of tasks in a project being monitored with a webhook? I’m pretty sure they didn’t used to when I started with Asana a few years ago, but they do now??