There are definitely still cases where warnings are important, but the point of "prefer undo to warnings" is to eliminate as many warnings as possible to avoid desensitizing users. I should only see a scary pop-up if the action that I'm performing is going to be well and truly destructive.

Undo-instead-of-warning is a great pattern for lower-stakes actions that are easy to undo and cause minimal damage if left done. This allows the few warnings you do show to be recognized as truly important.