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.