Where LiquidThreads is enabled it adds a "Discussions" search of odd-numbered namespaces to Special:Search's set of search profiles (T22395: Add "Discussions" tab to 'advanced search'). This seems a generally useful feature, I'm not sure why it's tied to LQT.
This is in place on mediawiki.org despite its wmgLiquidThreadsFrozen['mediawikiwiki'] => true, and surprisingly it still offers search results from old threads, which redirect to the migrated Flow topics. Try e.g. search "Discussions" for "Protection level of main page"
So it's useful, but searching discussions won't find new Flow topics (a known Flow limitation, T62493: StructuredDiscussions posts are not indexed in builtin search and T90082: Flow boards should appear in site search) on mediawiki.org's Flow-ified talk pages, either requested or that have been converted from LQT pages. Try e.g. search "Discussions" for "markdown supported", a recent topic.
I'm reporting oddness here for future generations ☺, but if mediawiki.org users still want a "Discussions" search profile, the current situation may be the best we can do. If they don't want it, the profile will only go away when LQT is no longer enabled or if LQT's customiseSearchProfiles() hook gets a config flag.