It was decided that Multibyte does not belong to as/mini.

activesupport/lib/active_support/mini.rb

 $LOAD_PATH.unshift File.dirname(__FILE__)
 
 require "core_ext/blank"
-# whole object.rb pulls up rare used introspection extensions
+# whole object.rb pulls up rarely used introspection extensions
 require "core_ext/object/metaclass"
 require 'core_ext/array'
 require 'core_ext/hash'
 require 'core_ext/module/attribute_accessors'
-require 'multibyte'
-require 'core_ext/string/multibyte'
-require 'core_ext/string/inflections'
-
-class String
-  include ActiveSupport::CoreExtensions::String::Multibyte
-end
+require 'core_ext/string/inflections'
\ No newline at end of file