For what it's worth, the issue seems to fix itself when "NOTOC" is added (see https://en.wikivoyage.org/w/index.php?title=UNESCO_World_Heritage_List&diff=prev&oldid=4840979).

@cscott I'm guessing this is fallout from the changes you were working on last week?

This can be achieved by adding the NOTOC keyword in the template that calls the banner. However, I am not sure if such a solution is really desirable, or something should be fixed inside the Mediawiki code instead.

Regardless of whether behaviour changed here, this does seem like the ideal solution here to me - I imagine some wikis may want both, or want only the table of contents in the sidebar.

@Jdlrobson unfortunately also in en:voy there's the need for some user to show/use the TOC, so it's necessary to solve the bug server-side, restoring the previous behavior.
Thanks for contacting @cscott or whomever could be involved on its resolution.

Change 1010334 had a related patch set uploaded (by C. Scott Ananian; author: C. Scott Ananian):

[mediawiki/extensions/WikidataPageBanner@master] Fix inversion error in `enable-toc` test

https://gerrit.wikimedia.org/r/1010334

We pinpointed this issue to https://gerrit.wikimedia.org/r/c/mediawiki/extensions/WikidataPageBanner/+/972060

Patch was reverted as a temporary measure so this will ride the train this week.