Hey folks :)

The in other projects sidebar is a feature that adds links to articles
on other projects to the sidebar. So for example on the Wikivoyage
article about Berlin it links to the respective content on Wikipedia,
Commons and so on.This works similar to the language links that you
already have in the sidebar of an article. It is intended to make more
content available to our users and bring our projects closer together.

The in other projects sidebar has been around for many months now. It
has been a beta feature on all wikis and default on a handful. It
can't stay a beta feature forever and we should decide on its faith
now. I am happy with the reactions we have been getting about it and
believe it adds significant value for our readers and editors on
smaller projects. I therefor want to enable it on all projects. This
will happen in January. The exact date is still being decided.

If for some reason your project decides it does not want this feature
please let me know and we'll disable it for you.

The ticket for tracking this is https://phabricator.wikimedia.org/T103102