Miraheze

Miraheze is a free, non-profit wiki hosting service that uses MediaWiki version 1.36.0 as its hosting engine, and commits to upgrade to the most recent stable version. Miraheze auto-installs several MediaWiki extensions on new wikis, with users have the option to enable many other extensions on the wiki (and request new ones), including VisualEditor and StructuredDiscussions. Miraheze is supported by user donations and does not include any advertising on wikis it hosts.

Miraheze currently hosts more than 4000 wikis.

Extensions
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.

Custom domains
If requested, Miraheze provides support for custom domains, as long as the domain is pointed to them as it is said here.

Code
Miraheze uses open-source code, and hosts it at GitHub.

Extensions, settings and permission management
Miraheze uses their own extension, called ManageWiki in order to allow users on individual wikis to enable/disable extensions, change a variety of settings (their logo, their favicon, etc.), change permissions and manage namespaces.

Issue reporting
Miraheze uses their own Phabricator for issue/bug reports as well as requesting some features for wikis.

Servers
Miraheze currently functions with 33 different servers used for different purposes (MediaWiki, Varnish, Database, Puppet, Test, Bacula, Gluster, etc.). A list of these servers and each of their functions can be seen here.