I had to do the same thing to stop my washer and dryer notifications from showing after every ha restart. The problem started in my automation, which used a state change node and checks if it is below 12W for a few seconds. Since the machines draw constant power while off, it only triggers once when the cycle ends. However, during reboot the states go to unavailable for a bit, then a new 0W state is sent, which triggers the notification.
I fixed this using a state trigger node. The allowed conditions include current and previous states are not unknown or unavailable, and current state is <12. Then I have a default output for previous state >=12, which feeds the rest of the notification flow. This way, nothing flows after a restart, and anytime the power drops below 12W (falling edge detect) the flow starts. I have a delay downstream to make sure it stays <12 for longer than 5sec (since the washer sometimes briefly dips below that during a cycle).
The event trigger nodes are very useful for this sort of thing… I was able to greatly simplify some of my flows by removing function nodes and replacing the if/else checks with conditions in the trigger node.