I see no 'nickname' reference in my Preference menu. It refers rather to 'Signature' in https://www.mediawiki.org/wiki/Special:Preferences#mw-prefsection-personal
It is defined in https://www.mediawiki.org/wiki/Manual:$wgDefaultUserOptions
Other nickname links of interest : Nick name is provided via {{#nickname:alt}} → https://www.mediawiki.org/wiki/Extension:UserFunctions/en