The beta feature "in other projects sidebar" has been around for a while. It adds a sidebar section to an article linking to the same topic on relevant sister projects. The data for it comes from Wikidata. It's been taken out of beta on a number of Wikipedias that requested it. We should get it out of beta everywhere and make it available for everyone.
Rationale: We want to give our readers access to more information in the other sister projects and we want to help our editing communities get closer together.
Instructions for sysops of wikis which previously implemented this as a gadget: update your code to build on top of Wikibase (e.g. https://it.wikipedia.org/wiki/MediaWiki:InterProject.js called on Common.js).