diff --git a/Modules/Core/Common/include/itkSingleton.h b/Modules/Core/Common/include/itkSingleton.h
index 7e06639aeca..17b939f9497 100644
--- a/Modules/Core/Common/include/itkSingleton.h
+++ b/Modules/Core/Common/include/itkSingleton.h
@@ -23,15 +23,18 @@
 #include <map>
 #include <functional>
 
+#ifndef ITK_FUTURE_LEGACY_REMOVE
 /** \brief A function which does nothing
+ * \deprecated Preferably use the C++ `[[maybe_unused]]` attribute instead!
  *
  * This function is to be used to mark parameters as unused to suppress
  * compiler warning. It can be used when the parameter needs to be named
  * (i.e. itkNotUsed cannot be used) but is not always used.
  */
 template <typename T>
-inline void
+[[deprecated("Preferably use the C++ `[[maybe_unused]]` attribute instead!")]] inline void
 Unused(const T &){};
+#endif
 
 namespace itk
 {
@@ -124,9 +127,8 @@ template <typename T>
 T *
 Singleton(const char * globalName, std::function<void()> deleteFunc)
 {
-  static SingletonIndex * singletonIndex = SingletonIndex::GetInstance();
-  Unused(singletonIndex);
-  T * instance = SingletonIndex::GetInstance()->GetGlobalInstance<T>(globalName);
+  [[maybe_unused]] static SingletonIndex * singletonIndex = SingletonIndex::GetInstance();
+  T *                                      instance = SingletonIndex::GetInstance()->GetGlobalInstance<T>(globalName);
   if (instance == nullptr)
   {
     instance = new T;
diff --git a/Modules/Core/Common/src/itkObjectFactoryBase.cxx b/Modules/Core/Common/src/itkObjectFactoryBase.cxx
index ce883086638..9c78f6f5dc1 100644
--- a/Modules/Core/Common/src/itkObjectFactoryBase.cxx
+++ b/Modules/Core/Common/src/itkObjectFactoryBase.cxx
@@ -832,8 +832,7 @@ ObjectFactoryBase::SynchronizeObjectFactoryBase(void * objectFactoryBasePrivate)
 std::list<ObjectFactoryBase *>
 ObjectFactoryBase::GetRegisteredFactories()
 {
-  //  static SingletonIndex * singletonIndex = SingletonIndex::GetInstance();
-  //  Unused(singletonIndex);
+  //  [[maybe_unused]] static SingletonIndex * singletonIndex = SingletonIndex::GetInstance();
   ObjectFactoryBase::Initialize();
   return m_PimplGlobals->m_RegisteredFactories;
 }