Add special rule for async void eventhandlers.

[JohanLarsson]

private async void OnChanged(object sender, EventArgs e)
{
    ...
}

What about adding a rule AvoidAsyncVoidInEventhandlers?
The reason for this would be to be able to tune severity separately for them.
Heuristics could be: method with two arguments of type object and assignable to EventArgs or maybe it can be figured out from climbing around the syntax tree that the method is only used for subscribing to an event.

Perhaps there could be another analyzer checking that there is try-catch surrounding all awaits.

[AArnott]

Given that async void methods crash the process if they throw, I don't think they should ever be used, event handlers or not.