To aid seamless transition for merging DismissableSiteNotice to core this variable should be deprecated. Ideally it should be even left in the transition in the interest of removing the overhead of not-so useful config variables and given that Stable Interface Policy considers such globals as unstable, nonetheless, that may seems too ambitious or radical. So going the route of deprecation.

It's hard to see the point of making banner undismissable in the present age and web standards.

That notwithstadning, this flag exists for whatever reasons, since we are merging this button to core, it's time to rethink it.
If a MediaWiki site admin intentionally needs to make the notice undismissable, they are free to use the SiteNoticeBefore hook to modify the notice and make it so. I am sure Wikimedia won't want do that for the foreseeable future.

Note; the variable needs to be added to core first, that will be done in T262118