Theres an implicit assumption that the actions being offered an “undo” are semi-important/permanent.

In other words, if you delete an email and it goes to the trash folder: good use of toast + undo

If you empty the trash, and there is nowhere the user can go to unempty it: bad use of toast + undo

Its also useful as a sleight of hand eg when cancelling an action you havent yet taken (which actually is generally what a toast + undo actually is). The best example of this being toast + undo for an email send

Putting an email in trash seems like exactly the kind of action that does _not_ need a toast, since it's easy and obvious to undo.