There are currently several hundreds extensions hosted on SVN https://svn.wikimedia.org/viewvc/mediawiki/trunk/extensions/ and many of them are still working according to www.mediawiki.org. However, a quick glance shows that many of them are overlapping each other and likely to be unmaintained. It should be of interest to the MediaWiki Cooperation https://www.mediawiki.org/wiki/MediaWiki_Cooperation group to merge extensions where appropriate and move them to git if still useful.