Please forgive if this isn't welcome, but I suspect in the current political climate in the USA it may very well be more likely to see in the short/medium term. Furthermore, I'm very concerned about the fact that, even though there's no evidence of happening yet, this would be a way for vandalism to introduce even potentially oversightable information into previews that, as far as I can see, there is no guaranteed way to remove quickly (and even if there is, most of us non-techy contributors have no idea what it is). Furthermore, previews are built in to much more than desktop - they're automatic in mobile (as far as I'm aware) for example - meaning it is highly visible even to logged-out/non-contributors. If nothing else, it should be a high priority to add a noticable and easy way to purge the previews for local contributors who revert an edit - ideally it would be automatic, but failing that a clear way to purge is needed.