Description
Event Timeline
It isn't. Enwiki uses a custom gadget, not an extension/core. You must import MediaWiki:Gadget-geonotice.js, MediaWiki:Gadget-geonotice, MediaWiki:Gadget-geonotice-core.js, MediaWiki:Gadget-geonotice-core, MediaWiki:Gadget-geonotice-list.js and MediaWIki:Gadget-geonotice-core.css from enwiki and add two lines to https://ar.wikipedia.org/wiki/%D9%85%D9%8A%D8%AF%D9%8A%D8%A7%D9%88%D9%8A%D9%83%D9%8A:Gadgets-definition (enwiki https://en.wikipedia.org/wiki/MediaWiki:Gadgets-definition). Then it should work.
Cheers,
Martin
You also need to adapt https://en.wikipedia.org/wiki/MediaWiki:Watchlist-messages into enwiki as well as https://en.wikipedia.org/wiki/MediaWiki:Watchlist-summary.
Thanks a lot @Urbanecm for the detailed explanation how other sites could enable this!