Per Brion's bug 9209 comment 4, making the sitenotice dismissable (and loaded via JavaScript) for unregistered users is one of the main requirements of this extension.
I don't know exactly at what point this stopped working, I noticed only now. It was reported in 2012 at [[mw:Extension_talk:DismissableSiteNotice#How_can_anons_dismiss_a_notice.3F]].
Version: unspecified
Severity: major