Right now, on fr-wp, you can select the "Sujet" namespace, although there is no content stored there. Instead it is a virtual namespace that was created for the Flow boards on french Wikipedia ( see also T202010#4522547). Since virtual namespaces never contain content, they should not be selectable for search
- The sujet namespace is not visible in fr-wp anymore, and no topic namespace on mediawiki.org
- According to T202010#4522547, this should be doable by filtering out the NS_TOPIC namespace (=2600)
- This can probably only be unit tested
- Find out if virtual namespaces are known to the client-side, move to server-side injection of namespace config into client-side code.