The design for T273073 calls for a dialog element which will appear when the user wants to select namespaces for a search query (the large number of options makes other ways of displaying this info unfeasible).
We don't yet have a Vue-based UI component for this kind of dialog, so we'll need to create one before the filter-by-namespace feature can be completed.
One additional hurdle is that Vector skin currently makes non-OOUI dialogs very difficult to do; the skin styles are written in such a way that creating a full-screen overlay is not really possible without using OOUI's WindowManager component.
In the SuggestedTags interface, we actually ended up relying on OOUI for the dialogs while using Vue for everything else. We'd like to avoid doing that on MediaSearch because it will necessitate keeping both libraries on the page. However, as a short-term workaround this might be acceptable if we need to get this feature working in time for the default rollout.