Manual:$wgDismissableSiteNoticeForAnons

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Miscellaneous settings: $wgDismissableSiteNoticeForAnons
Whether site notice dismiss button "[dismiss]" should be shown to anonymous (unregistered) users.
Introduced in version:1.36.0 (Gerrit change 631177; git #14ce139f)
Removed in version:1.36.0 (Gerrit change 654460; git #34ab93a6)
Allowed values:(boolean)
Default value:false

DetailsEdit

Whether site notice dismiss button "[dismiss]" should be shown to anonymous (unregistered) users. If it's false (default), sitenotices are effectively undismissable unless one registers.

The button will be shown to all registered users irrespective of this setting.