enricodx it's a different situation for discussions or posts.
For discussions, you can search for is:hidden
in the search bar and it will return all hidden discussions that you can see, for example https://discuss.flarum.org/?q=is:hidden (you probably won't see anything unless you have deleted discussions of your own).
For posts however there is no such thing at this time. Both the REST API doesn't have the ability to return only hidden posts, and there's no graphical interface that would be able to show it anyway.
Coming with Flarum beta 16, the GET /api/posts
REST endpoint will have customizable filters (right now it isn't extendable), so an extension will be able to register a hidden post filter. You'll still need to build some kind of interface to show a list of posts outside of a discussion though.
That being said, there isn't really a reason to permanently delete discussions/posts. Even if there's sensitive/personal information in a post, you might want to just edit that information out and keep the remaining of the hidden post in case moderators ever need to find out what happened in the past.