It's the most annoying thing I felt when I using software too. So in my own project, I tend to just keep the message open and wait for user to decide what do to with it, but then that's not a toast anymore.

I don't think designers should put anything interactive in an arbitrarily timed interface aside from "Dismiss". A toast is the best when it's displaying what is currently going on, not as a pop up dialog box.

The best design for Undo I think is to make it a dedicated button, like the one in the text editors. When user clicked "Archive", a Toast pop up and displays message "Archiving N entries, please wait" and then change it to "N entries archived. You can press Control+C or click [Undo Icon] to undo if that was a mistake" then the Undo button lights up.

Also, IMO the message format "Archiving N entries, please wait" should be a standard, it tells the user in a clear way 1) what the software is doing, and 2) what should I the user do. On the other hand, the message "Conversation archived" don't really provide the same value, since user already saw it happened.