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:
https://github.com/discourse/discourse/commit/8dfb1be4d1912e44fece17b80f12d952d843a3c3
Note:
If you need to securely hide a topic you should convert it to a PM or move to a secure category.