fcitx5-qt is the Qt im-module for fcitx5 and it needs to be installed
in order to use fcitx5 with Qt-based applications. It currently
supports Qt4, Qt5 and Qt6.

You'll need several Qt modules if you're going to enable Qt5/Qt6
support, namely QtCore, QtDbus, QtWidgets and QtX11Extras. Qt5 is
bundled in Slackware, however if you want Qt6 support you're on your
own; but any app that requires Qt6 on SBo at this point should have
already pulled all the dependencies in. 

Relevant CMake options are ENABLE_QT4, ENABLE_QT5 and ENABLE_QT6.
ENABLE_QT4 is turned off by default, ENABLE_QT5 and ENABLE_QT6 are
turned on by default. However to avoid pulling in excess dependencies,
Qt6 support is turned off in the SlackBuild. If you need Qt4 or Qt6
support, you might want to turn the flags on/off depending on the Qt
version you are using. To enable Qt4, you need to modify

  -DENABLE_QT4=Off \

from "Off" to "On". Similarly, to turn on Qt6 support, turn

  -DENABLE_QT6=Off \
  -DENABLE_QT6_WAYLAND_WORKAROUND=Off ..

from "Off" to "On".