Miraheze is a free, non-profit wiki hosting service that uses MediaWiki as its hosting engine, it provides any extension of your choice, including VisualEditor and Flow. Miraheze is not advertising supported, and does not have advertising on wikis it hosts.
Miraheze can download any extension of your wish and enable it on your wiki after 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 it's GitHub repository.
Miraheze uses their own Phabricator for issue/bug reports as well as requesting features for wikis.