Now it depends against new version. Old code can be removed

src/platformtheme/kdeplatformfiledialoghelper.cpp

@@ -424,12 +424,6 @@ QUrl KDEPlatformFileDialogHelper::directory() const
 
 void KDEPlatformFileDialogHelper::selectFile(const QUrl &filename)
 {
-    // This is called once by QFileDialogPrivate::init -> QFileDialog::selectUrl -> QFileDialogPrivate::selectFile_sys
-    // and then again by selectFile in the QFileDialog constructor, with a wrong value for remote URLs, until the Qt 5.11.2 fix.
-#if QT_VERSION < QT_VERSION_CHECK(5, 12, 0)
-    if (m_fileSelected && !m_dialogInitialized)
-        return;
-#endif
     m_dialog->selectFile(filename);
     m_fileSelected = true;
 }

src/platformtheme/kdirselectdialog.cpp

@@ -147,11 +147,7 @@ void KDirSelectDialog::Private::slotMkdir()
     bool exists = false;
     QUrl folderurl(m_parent->url());
 
-#if QT_VERSION < QT_VERSION_CHECK(5, 15, 0)
-    const QStringList dirs = directory.split(QLatin1Char('/'), QString::SkipEmptyParts);
-#else
     const QStringList dirs = directory.split(QLatin1Char('/'), Qt::SkipEmptyParts);
-#endif
     QStringList::ConstIterator it = dirs.begin();
 
     for (; it != dirs.end(); ++it) {