[deleted] I don't know what would happen. But this extension certainly won't prevent you to set dates in the future.
I don't think Flarum itself will encounter issues if you do it, but it could mess with some other community extensions that do things based on the discussion date.
If the discussions has no replies, it might be impossible to mark it as read if it has a date in the future.
Changing the date of posts can cause more serious issues however. When the numbers and dates are no longer chronological, linking to a specific post can cause all kind of weird jumping issues.
Permanently sticking posts could be achieved via an extension as is said in the other discussions about it. Personally I only see that change as bad UX. The homepage is supposed to show content the user hasn't read yet.