callback_deprecation_warning.rb

      def display_deprecation_warning_for_false_terminator
        ActiveSupport::Deprecation.warn(<<-MSG.squish)
          Returning `false` in Active Record and Active Model callbacks will not implicitly halt a callback chain in Rails 5.1.
          To explicitly halt the callback chain, please use `throw :abort` instead.
        MSG
      end

Originally from https://github.com/rails/rails/blob/c4d3e202e10ae627b3b9c34498afb45450652421/activesupport/lib/active_support/callbacks.rb#L783-L788