fcitx5-qt is the Qt im-module for fcitx5 and it's needed 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.
Both ENABLE_QT4 and ENABLE_QT5 are turned on by default, and ENABLE_QT6
is turned off by default; however Slackware has dropped Qt4 in Slackware
15, so ENABLE_QT4 is turned off in the SlackBuild. You might want to
turn on/off the flag depending on the Qt version you are using.