They say to replace them with Banners, which are just a different style of a "Toaster", just usually stay longer, or are permanent until the user takes an action.
The main problem with toasts is that they disappear with no hope of recovery before you get a chance to read them, obscuring other content in the process. Banners don't obscure other content, don't disappear without user action, and could theoretically have a message history.