I was surprised that only this week I landed my first code contribution to MediaWiki. I would have sworn that I did so before, but I found nothing in git history.
MediaWiki is the 20 years old software running many widely used wikis, eg Wikipedia, Wikidata, Commons, Fandom, written mostly in PHP. Per GitHub, it saw more than 112,000 patches by almost 600 contributors. It is the core of an ecosystem of hundreds of extensions.