KDELibs4Support kdelibs4support/src/solid-networkstatus/kded/CMakeLists.txt relies upon cmake/Qt5DBus/Qt5DBusMacros.cmake to define qt5_add_dbus_adaptor. That file does not exist in Qt6. The remaining two files in cmake/Qt5DBUS do not define qt5_add_dbus_adaptor, so CMake throws an error: "Unknown CMake command "qt5_add_dbus_adaptor". Just a heads-up, kdelibs4support build will break when Qt releases the next major version in Dec 2020. Maybe you could simply copy the Macro definition from Qt5DBusMacros.cmake into CMakelists.txt?
kdelibs4support will not be ported to Qt6 so this should be a non-issue