feature_flag.rb

require 'pry'

$v2 = true

module V2
  def self.included(base)
    base.extend(ClassMethods)
  end

  module ClassMethods
    def v2_feature
      yield if $v2
    end
  end
end

class Foo
  include V2

  def bar
    "I'm a V1 feature"
  end

  v2_feature do
    def baz
      "I'm a V2 feature"
    end
    
    def call_private_baz
      private_baz
    end
    
    private
    
    def private_baz
      "I'm a private V2 feature"
    end
  end
end

foo = Foo.new
foo.bar
#=> "I'm a V1 feature"
foo.baz
#=> "I'm a V2 feature"
foo.private_baz
#=> NoMethodError: private method `private_baz' called for #<Foo:0x007ff3029f2ea8>
foo.call_private_baz
#=> I'm a hidden V2 feature

$v2 = false # The file must be reloaded for this to take effect in the V2 module

foo = Foo.new
foo.bar
#=> "I'm a V1 feature"
foo.baz
#=> NoMethodError: undefined method `baz' for #<Foo:0x007fb824105728>