Backup files?
You can take a complete archive via: OPTIONS > DOWNLOAD ARCHIVES but I doubt that's what you want as it is not the simple "Undo" feature that I'm guessing you seek, and there's no simple way to re-post a message found within the backup.
It does depend what you are seeking to delete. If you're at the forum/sub-forum level then you really do need to understand the Nabble jargon to know what you are about to delete. (That's what the warning is there for, after all!)
If you are at the thread and individual post level, then it is probably better to move the post to a hidden sub-forum (i.e. one that doesn't have "view" permissions to ordinary members. I keep one called "Moderated Posts" for the purpose.)
Be aware that when you move any post and all replies move with it! Once you have checked that everything you intended to be removed from the original locations has gone, then you can delete it from the Moderated Posts sub-forum, otherwise move it back and try a different approach.
Volunteer Helper - but recommending that users move off the platform!
Once the admin for GregHelp now deleted.