Manual:$wgUseMediaWikiUIEverywhere

Output: $wgUseMediaWikiUIEverywhere
Applies MediaWiki UI wherever it can be supported.
Introduced in version:1.24.0 (Gerrit change 150635; git #aa15d528)
Removed in version:1.42.0 (Gerrit change 995376; git #be135c0f)
Allowed values:(boolean)
Default value:false

Details edit

Setting this to true will enable MediaWiki UI wherever it can be supported.

This variable is temporary only and will be removed, once MediaWiki UI is more stable.
MediaWiki UI is deprecated in favour of Codex.