Added new logic for handling hiding of expression variables, this will help to have a clear way of not showing variables that are not needed anymore but they can break past QGIS projects if they are removed.

python/core/auto_generated/qgsexpressioncontext.sip.in

@@ -404,6 +404,63 @@ Writes scope variables to an XML ``element``.
 .. versionadded:: 3.6
 %End
 
+
+    QStringList hiddenVariables() const;
+%Docstring
+Returns the list of variables hidden within the scope.
+
+.. seealso:: :py:func:`setHiddenVariables`
+
+.. seealso:: :py:func:`addHiddenVariable`
+
+.. seealso:: :py:func:`removeHiddenVariable`
+
+.. versionadded:: 3.28
+%End
+
+    void setHiddenVariables( const QStringList &hiddenVariables );
+%Docstring
+
+Sets the list of variables intended to be hidden.
+
+.. seealso:: :py:func:`hiddenVariables`
+
+.. seealso:: :py:func:`addHiddenVariable`
+
+.. seealso:: :py:func:`removeHiddenVariable`
+
+.. versionadded:: 3.28
+%End
+
+
+    void addHiddenVariable( const QString &hiddenVariable );
+%Docstring
+
+Adds the passed variable to a list of hidden variables.
+
+.. seealso:: :py:func:`hiddenVariables`
+
+.. seealso:: :py:func:`setHiddenVariables`
+
+.. seealso:: :py:func:`removeHiddenVariable`
+
+.. versionadded:: 3.28
+%End
+
+    void removeHiddenVariable( const QString &hiddenVariable );
+%Docstring
+
+Removes the passed variable from a list of hidden variables.
+
+.. seealso:: :py:func:`hiddenVariables`
+
+.. seealso:: :py:func:`setHiddenVariables`
+
+.. seealso:: :py:func:`addHiddenVariable`
+
+.. versionadded:: 3.28
+%End
+
 };
 
 class QgsExpressionContext

src/core/qgsexpressioncontext.cpp

@@ -47,6 +47,7 @@ QgsExpressionContextScope::QgsExpressionContextScope( const QgsExpressionContext
   , mFeature( other.mFeature )
   , mHasGeometry( other.mHasGeometry )
   , mGeometry( other.mGeometry )
+  , mHiddenVariables( other.mHiddenVariables )
 {
   QHash<QString, QgsScopedExpressionFunction * >::const_iterator it = other.mFunctions.constBegin();
   for ( ; it != other.mFunctions.constEnd(); ++it )
@@ -63,6 +64,7 @@ QgsExpressionContextScope &QgsExpressionContextScope::operator=( const QgsExpres
   mFeature = other.mFeature;
   mHasGeometry = other.mHasGeometry;
   mGeometry = other.mGeometry;
+  mHiddenVariables = other.mHiddenVariables;
 
   qDeleteAll( mFunctions );
   mFunctions.clear();
@@ -120,6 +122,29 @@ QStringList QgsExpressionContextScope::variableNames() const
   return names;
 }
 
+QStringList QgsExpressionContextScope::hiddenVariables() const
+{
+  return mHiddenVariables;
+}
+
+void QgsExpressionContextScope::setHiddenVariables( const QStringList &hiddenVariables )
+{
+  mHiddenVariables = hiddenVariables;
+}
+
+void QgsExpressionContextScope::addHiddenVariable( const QString &hiddenVariable )
+{
+  if ( !mHiddenVariables.contains( hiddenVariable ) )
+    mHiddenVariables << hiddenVariable;
+}
+
+void QgsExpressionContextScope::removeHiddenVariable( const QString &hiddenVariable )
+{
+  if ( mHiddenVariables.contains( hiddenVariable ) )
+    mHiddenVariables.removeAt( mHiddenVariables.indexOf( hiddenVariable ) );
+}
+
+
 /// @cond PRIVATE
 class QgsExpressionContextVariableCompare
 {
@@ -208,8 +233,14 @@ void QgsExpressionContextScope::readXml( const QDomElement &element, const QgsRe
   {
     const QDomElement variableElement = variablesNodeList.at( i ).toElement();
     const QString key = variableElement.attribute( QStringLiteral( "name" ) );
-    const QVariant value = QgsXmlUtils::readVariant( variableElement.firstChildElement( QStringLiteral( "Option" ) ) );
-    setVariable( key, value );
+    if ( variableElement.tagName() == QStringLiteral( "Variable" ) )
+    {
+      const QVariant value = QgsXmlUtils::readVariant( variableElement.firstChildElement( QStringLiteral( "Option" ) ) );
+      setVariable( key, value );
+    }
+    else
+      addHiddenVariable( key );
+
   }
 }
 
@@ -223,6 +254,13 @@ bool QgsExpressionContextScope::writeXml( QDomElement &element, QDomDocument &do
     varElem.appendChild( valueElem );
     element.appendChild( varElem );
   }
+
+  for ( QString hiddenVariable : mHiddenVariables )
+  {
+    QDomElement varElem = document.createElement( QStringLiteral( "HiddenVariable" ) );
+    varElem.setAttribute( QStringLiteral( "name" ), hiddenVariable );
+    element.appendChild( varElem );
+  }
   return true;
 }
 
@@ -421,10 +459,20 @@ QStringList QgsExpressionContext::filteredVariableNames() const
   QStringList allVariables = variableNames();
   QStringList filtered;
   const auto constAllVariables = allVariables;
+
+  QStringList hiddenVariables;
+
+  for ( const QgsExpressionContextScope *scope : mStack )
+  {
+    const QStringList scopeHiddenVariables = scope->hiddenVariables();
+    for ( const QString &name : scopeHiddenVariables )
+      hiddenVariables << name ;
+  }
+
   for ( const QString &variable : constAllVariables )
   {
     if ( variable.startsWith( '_' ) ||
-         variable.compare( QStringLiteral( "frame_timestep_unit" ) ) == 0 )
+         hiddenVariables.contains( variable ) )
       continue;
 
     filtered << variable;

src/core/qgsexpressioncontext.h

@@ -380,6 +380,51 @@ class CORE_EXPORT QgsExpressionContextScope
      */
     bool writeXml( QDomElement &element, QDomDocument &document, const QgsReadWriteContext &context ) const;
 
+
+    /**
+     * Returns the list of variables hidden within the scope.
+     *
+     * \see setHiddenVariables()
+     * \see addHiddenVariable()
+     * \see removeHiddenVariable()
+     * \since QGIS 3.28
+     */
+    QStringList hiddenVariables() const;
+
+    /**
+     *
+     * Sets the list of variables intended to be hidden.
+     *
+     * \see hiddenVariables()
+     * \see addHiddenVariable()
+     * \see removeHiddenVariable()
+     * \since QGIS 3.28
+     */
+    void setHiddenVariables( const QStringList &hiddenVariables );
+
+
+    /**
+     *
+     * Adds the passed variable to a list of hidden variables.
+     *
+     * \see hiddenVariables()
+     * \see setHiddenVariables()
+     * \see removeHiddenVariable()
+     * \since QGIS 3.28
+     */
+    void addHiddenVariable( const QString &hiddenVariable );
+
+    /**
+     *
+     * Removes the passed variable from a list of hidden variables.
+     *
+     * \see hiddenVariables()
+     * \see setHiddenVariables()
+     * \see addHiddenVariable()
+     * \since QGIS 3.28
+     */
+    void removeHiddenVariable( const QString &hiddenVariable );
+
   private:
     QString mName;
     QHash<QString, StaticVariable> mVariables;
@@ -388,6 +433,7 @@ class CORE_EXPORT QgsExpressionContextScope
     QgsFeature mFeature;
     bool mHasGeometry = false;
     QgsGeometry mGeometry;
+    QStringList mHiddenVariables;
 };
 
 /**

src/core/qgstemporalnavigationobject.cpp

@@ -82,6 +82,9 @@ QgsExpressionContextScope *QgsTemporalNavigationObject::createExpressionContextS
   scope->setVariable( QStringLiteral( "animation_start_time" ), mTemporalExtents.begin(), true );
   scope->setVariable( QStringLiteral( "animation_end_time" ), mTemporalExtents.end(), true );
   scope->setVariable( QStringLiteral( "animation_interval" ), mTemporalExtents.end() - mTemporalExtents.begin(), true );
+
+  scope->addHiddenVariable( QStringLiteral( "frame_timestep_unit" ) );
+
   return scope.release();
 }