You can offer Undo via things other than toasts, though. In fact, I wish more software offered Undo--the Undo feature has kind of gone out of fashion since the early 2000s. You should be able to Undo anything (and follow the Undo chain back through many past actions). We somehow lost this ability from software.

For sure you can, but a toast is a tool in the toolbox. When it makes sense to use it, it definitely justifies its existence as a tool to keep around.

Undo's (and especially re-do) are quite hard and resource intensive to code, especially for web apps that can be simultaneously accessed via multiple devices.

E.g. you can take action A on your laptop, followed by action B on your phone. Undoing action A may not be easily possible if it was followed by action B.

To make that work properly you need to activel sync states between all the users devices using e.g. websocket or what-have-you. Handling edge cases becomes quite the nightmare, e.g. phone has poor connectivity.

Only the big guys would have budget to do these sort of things (And make them work well).

I've accidently archived something only to realize it when the toast pops up. I'm grateful for the toast instead of having the 'are you sure' like you mentioned. It's a nice compromise.

Undo via ephemeral toast seems like a bad idea also. I guess if the toast hanged around until the next toast it might make sense?

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.