It's the same with "Special:", right? Probably all namespaces with negative IDs should not be shown.
@Catrope and @SBisson, is this correct? Is Special also an unnecessary, bogus or redundant namespace? In my tests just now it returns results that are all logged actions, I think. Should we remove it?
Hmm, only if you search for log entries; there are some weird interactions which may or may not be intentional (e.g. all abuse filter modifications have "Special:AbuseFilter" as the page title). This could be useful in some cases, I guess, but I think using the form on Special:Log is a better way to search for those. There wasn't a "Special:" option in the old interface (non-RCFilters).
Checked in betalabs - Media namespace is removed from Namespace list. It seems that :Special returns only specific logged entries - Edit filter modification log and 'Deletion log' (actions related to changed visibility of a logged event). Overall, the :Special filter seems useful - it returns the subset of Logged events.
QA Recommendation: Resolve
I agree with @Etonkovidova.
Although it's a special case, these are valid results (although the edit filters arguably should use a real namespace and content model, that's a bigger change for another day), and I think it's valid to allow showing only Special (or inverting to exclude Special).