I would like to now if it is possible for regular users to soft-delete posts from one particular sub-forum ? Not for the complete forum, but only a part of it.
Forum permissions is the easiest way to allow users to delete. Then in the usergroup tab you will need to edit the remove stats so they only have soft delete.
If you want only specific people this would basically be done like adding new moderators. Just add a sub group & add them to that group with the appropriate permissions =)