Special:Contributions, Special:DeletedContributions, Special:NewPages, and some other special pages allow to show results either in all namespaces, or in one of the namespaces enabled, or (using Invert selection) in all namespaces except the selected one. But it might be sometimes needed to view the user's contribs, for example, in all talk namespaces. Instead of a dropdown box, introduce checkboxes for namespaces.

According to New Filters for Edit Review (if enabled), on pages like Recent Changes, Watchlist and Related Changes, you can select multiple namespaces to search by.