Why some repository readme shows a sticky button to show TOC of the readme content but most repository don’t have

See GitHub - rails/rails: Ruby on Rails for the button

How do I get this button for my repo ?

There is a Table of contents feature in the repo settings:

However, it seems it’s on the beta, i.e., not generally available yet.

Thanks, I thought that was a hidden feature…… seems like I’m not in the beta plan :smile:

Seems the feature is opened for every one