Grabbing LabPlot from AUR (Arch Linux) the compilaton issues some deprecation warnings that might be fixable (maybe they are already obsolete since LabPlot 2.5 won't support Qt 4 anymore):

[ 17%] Building CXX object src/CMakeFiles/labplot2.dir/kdefrontend/HistoryDialog.cpp.o
/tmp/pamac-build-usti/labplot-kf5/src/labplot-2.4.0-kf5/src/kdefrontend/HistoryDialog.cpp: In constructor ‘HistoryDialog::HistoryDialog(QWidget*, QUndoStack*, const QString&)’:
/tmp/pamac-build-usti/labplot-kf5/src/labplot-2.4.0-kf5/src/kdefrontend/HistoryDialog.cpp:41:129: warning: ‘KDialog::KDialog(QWidget*, Qt::WindowFlags)’ is deprecated [-Wdeprecated-declarations]
 HistoryDialog::HistoryDialog(QWidget* parent, QUndoStack* stack, const QString& emptyLabel) : KDialog(parent), m_undoStack(stack) {
                                                                                                                                 ^
In file included from /usr/include/KF5/KDELibs4Support/KDE/KDialog:1:0,
                 from /tmp/pamac-build-usti/labplot-kf5/src/labplot-2.4.0-kf5/src/kdefrontend/HistoryDialog.h:33,
                 from /tmp/pamac-build-usti/labplot-kf5/src/labplot-2.4.0-kf5/src/kdefrontend/HistoryDialog.cpp:28:
/usr/include/KF5/KDELibs4Support/kdialog.h:170:41: note: declared here
     KDELIBS4SUPPORT_DEPRECATED explicit KDialog(QWidget *parent = nullptr, Qt::WindowFlags flags = nullptr);
                                         ^~~~~~~

----------------------------------------------------------

 21%] Building CXX object src/CMakeFiles/labplot2.dir/kdefrontend/datasources/ImportFileDialog.cpp.o
/tmp/pamac-build-usti/labplot-kf5/src/labplot-2.4.0-kf5/src/kdefrontend/datasources/ImportFileDialog.cpp: In constructor ‘ImportFileDialog::ImportFileDialog(MainWin*, bool, const QString&)’:
/tmp/pamac-build-usti/labplot-kf5/src/labplot-2.4.0-kf5/src/kdefrontend/datasources/ImportFileDialog.cpp:62:75: warning: ‘KDialog::KDialog(QWidget*, Qt::WindowFlags)’ is deprecated [-Wdeprecated-declarations]
  cbAddTo(0), cbPosition(0), m_showOptions(false), m_newDataContainerMenu(0) {
                                                                           ^
In file included from /usr/include/KF5/KDELibs4Support/KDE/KDialog:1:0,
                 from /tmp/pamac-build-usti/labplot-kf5/src/labplot-2.4.0-kf5/src/kdefrontend/datasources/ImportFileDialog.h:33,
                 from /tmp/pamac-build-usti/labplot-kf5/src/labplot-2.4.0-kf5/src/kdefrontend/datasources/ImportFileDialog.cpp:30:
/usr/include/KF5/KDELibs4Support/kdialog.h:170:41: note: declared here
     KDELIBS4SUPPORT_DEPRECATED explicit KDialog(QWidget *parent = nullptr, Qt::WindowFlags flags = nullptr);
                                         ^~~~~~~

In fact there are dozens of the same deprecation warnings.