If a Battle.net friend logs on and off, I get two toasts, despite having done my best to disable these notifications as follows:
From the PC Desktop battle.net launcher, go to Settings → Notifications → uncheck every box. Settings → Friends & Chat → uncheck every box.
This is unnecessary spam, and there should be a way to stop it.
Edit: My bad! There’s a setting in WoW for this: Game Menu → Interface → Social, then uncheck “Online Friends” and “Offline Friends”.