Does anyone know how to replicate the English Wiki's pop-up page to protest against internet privacy.
Is there an extension or internal feature that allows for this?
Topic on Project:Support desk
I think it's Extension:DismissableSiteNotice
You don't need Extension:DismissableSiteNotice.
Wikipedia used Extension:CentralNotice to distribute it from the central wiki, however you don't need that extension either.
The notice system itself is in MediaWiki core. Edit MediaWiki:Sitenotice on your wiki and put a the HTML for the overlay in there with CSS classes on them, and from MediaWiki:Common.css style them to be fixed positioned over the entire page and use display: none; to hide everything else.