Noted that the module is new in version 2.5.

Doc/lib/libfunctional.tex

@@ -9,6 +9,7 @@
 
 \modulesynopsis{Higher-order functions and operations on callable objects.}
 
+\versionadded{2.5}
 
 The \module{functional} module is for higher-order functions: functions
 that act on or return other functions. In general, any callable object can