After some feedback and internal discussion we decided that the whole concept of “expecting slug matches” was deeply flawed since we expose the slugs anyway via JSON api.
Now “Unlisted” simply means (1) not visible in topic lists (except for staff) and (2) not google indexable. A lot easier to explain!
This is change is now implemented per:
If you need to securely hide a topic you should convert it to a PM or move to a secure category.