On dewp exist unused namespaces where currently are no pages in it (Gadget, Gadget Discussion, Gadget-Definition, Gadget-Definition Discussion and Thema).
As there is nothing to search it would be useful to hide those no content namespaces in the drop-menu of the search form.
The technology to do this exists. There is `SiteStats::pagesInNs()` in PHP, used for e.g. the `{{PAGESINNAMESPACE:…}}` #parserfunctions as well as in #mediawiki-extensions-scribunto. While none of the existing use cases run this in a loop, doing so should be fine. The results are cached via [main WAN cache](https://www.mediawiki.org/wiki/Manual:Object_cache#WAN_cache) for 1 hour.
Baseline:
[] Remove namespaces from the array that's send to the client when `SiteStats::pagesInNs()` returns zero. It's just noise that can't be used for anything anyway.
To be discussed:
[] Is it helpful to see that a namespace exists? If so, is it relevant that this happens in this particular place?