Manual:$wgReferrerPolicy

Details
String or array (since MediaWiki 1.31) or false.

Setting it to false prevents any the meta tag from being output.

Since MediaWiki 1.31, setting it to an array means the first value will be the default, and the others serve as a fallback chain for browsers which do not understand it. This will result in multiple meta tags being output, in the reverse order, as last valid tag takes precedence according to the specification. Previous versions of MediaWiki did not support an array value for this setting, causing a fatal error if an array is used.