You can't have missed it — Notion now has a floating table of contents enabled by default on all pages. You can still add an in-page table of contents (use the shortcut /toc), but this new model makes it more discreet and is always with you as you scroll down your page.

You can disable/enable it on any page by:

1. Clicking the ••• menu at the top-right of your page
2. Switching the "Table of contents" toggle on/off

‍