Given the following class subjects_xml and subjects_html are public, not protected.

class Blog
  def self.available_subjects(format)
    case format
    when :xml then subjects_xml
    when :html then subjects_html
    end
  end

  protected
  def self.subjects_xml
    [xml implementation]
  end

  def self.subjects_html
    [html implementation]
  end
end

The def self. code can be thought of as defining a class method of Blog, but it is actually defining a method on self, where self is the virtual class of Blog. The protected visibility modifier is being used to specify the visibility of the methods of Blog. Since the subjects_xml and subjects_html methods are being defined on self (the virtual class of Blog) the protected visibility is not applied.

It is possible to define protected class methods in various ways.

class Blog
  class << self
    def available_subjects(format)
      case format
        when :xml then subjects_xml
        when :html then subjects_html
      end
    end

    protected
    def subjects_xml
      [xml implementation]
    end

    def subjects_html
      [html implementation]
    end
  end
end

class Blog
  def self.available_subjects(format)
    case format
    when :xml then subjects_xml
    when :html then subjects_html
    end
  end

  def self.subjects_xml
    [xml implementation]
  end

  def self.subjects_html
    [html implementation]
  end
  
  class<<self;self;end.send :protected, :subjects_xml, :subjects_html
end