Miraheze is a free, non-profit wiki hosting service that uses MediaWiki version 1.30 as its hosting engine, and commits to upgrade to the most recent stable version. Miraheze auto-installs several MediaWiki extensions on new wikis, and will install most extensions at the request of the user who set up the wiki, including VisualEditor and Flow. Miraheze is supported by user donations and does not include any advertising on wikis it hosts.
Miraheze can download any MediaWiki extension, at the request of the operator of the wiki, provided that it passes a security review. Many of these can be found at Wikimedia's Git code repositories. To see extensions that are currently downloaded on Miraheze see their github.
There are also Extensions by Miraheze, available as free open source software.
If requested, Miraheze provides support for custom domains, but under some conditions.
Miraheze uses open-source code, and hosts it at GitHub.
Miraheze uses their own Phabricator for issue/bug reports as well as requesting features for wikis.
Miraheze currently functions with 15 different servers used for different purposes (MediaWiki, Varnish, Database, Puppet, etc.). A list of these servers and each of their functions can be seen here.