Toasts are like a tourniquet, for me. You slap them on a problem to stop the bleeding, and they're often preferable to the alternative, but nobody actually likes them and you really shouldn't keep them long-term or apply them frivolously.
E.g. you join a project with little to no error handling or success feedback. Slap some default toasts on everything! And then remove them page by page as you fix the horrible UI you inherited.
GitHub is large enough that they don't benefit from or want such a crutch, but not everyone is GitHub.