This feature is available to all users of Trust Level 1 (Basic Member) and above. A Table of Contents can only be inserted into the first post of any topic.
Hopefully this helps with readability for folks making megaposts or guides, and we hope to be able to utilize this feature in the future as well (like for patch notes).
For an example of a table of contents post with lots of stuff in it, here’s this example using Secret World Legends’ patch notes: Test post for table of contents