It seems that some admins prefer SiteNotice to CentralNotice, because it's the only way to target messages to a specific wiki.

Make sure CentralNotice can do anything SiteNotice can, so there's less reason to use duplicate and unmaintained code. SiteNotice can deliver a snippet of HTML on every pageview to a specific wiki, but currently CentralNotice can only target interface language, which is a totally different thing and will result in banners being delivered on wikis in every content language, when the interface language matches.

I believe that this workaround will no longer necessary if we can find an acceptable compromise for T53475. I don't want to argue for deprecating SiteNotice at that point however, cos it's probably still useful as a backup system.