By Alvin Alexander. Last updated: May 9, 2016
The page I linked to shows what “table of contents” modules work with Drupal 8. The reality as of May 9, 2016, is that only “TOC Filter” works. It relies on the “TOC API” module, which should not really be shown in that list, as it’s just a support module.
“TOC Formatter” claims to work with Drupal 8, but I could not get it to work, and its page shows that it has no Drupal 8 installations, so something smells fishy there.
This image shows what that page shows as of this date:
