Quote from https://gerrit.wikimedia.org/r/372515: "Changes in the behavior: [...] The state of the button is not saved in a cookie."

@Schnark So, what is it based on? Technical unfeasibility? Perceived lack of necessity? There are users who are not happy about it. I think, some would say, there is no need in the hiding/showing functionality at all if the state is not saved in a cookie. The only thing it does when used on one page is saves a small amount of space.

Don't know, let's ask @Fomafix.

It is possible to reimplement the former behavior and store the status in a cookie on a change. Restoring the status from a cookie with JavaScript can only be done after DOM ready and this leads to a flash of the full ToC before it get minimized. A way to avoid this is to hide the ToC already on server-side. With a cookie this would also be possible but cookies are unwanted for such a think. An alternative is to store the status in a user option. Anonymous user have no user options and therefor they can not store the status in the user options. An advantage of storing in user options is that the status is shared for browser the user uses.

Change 490705 had a related patch set uploaded (by Fomafix; owner: Fomafix):
[mediawiki/core@master] ToC: Save and restore the state of the toc toggle in a cookie

https://gerrit.wikimedia.org/r/490705