According to the patch 616f8ccf (see [1]), the wpad-detector feature (which uses Notifications) is optional, but the main CMakeLists.txt lists "Notifications" as a required KF6 component. [1] https://invent.kde.org/network/kio-extras/-/commit/616f8ccf5f6816ae4577ecf875aab6219452a2e6
Fixed with https://invent.kde.org/network/kio-extras/-/commit/f05381561f1b4a988c46dd7c621e9446e0e08529