When must delegated airspace be listed in the AIP?
Principles under ICAO agreements

Delegation of ATC services is contracted between ICAO members using bilateral or multilateral letters of agreement. Cases of such delegation exist typically to cover remote large airspace (e.g. Arctic agreement between multiple countries), or small areas (e.g. islands).

ICAO members are committed to fulfill the decisions of their constituent act, the Chicago Convention signed in 1944. Among the annexes to the Convention, annex 15 is related to the provision of aeronautical information services (AIS):


  Annex 15 defines how an aeronautical information service shall receive
  and/or originate, collate or assemble, edit, format, publish/store and
  distribute specified aeronautical information/data. The goal is to
  satisfy the need for uniformity and consistency in the provision of
  aeronautical information/data that is required for the operational use
  by international civil aviation.


Impact on AIP

Annex 15 includes these generic articles:


  3.1.1.2 Each Contracting State shall take all necessary measures to 
  ensure that the aeronautical information/data it provides relating to
  its own territory, as well as areas in which the State is responsible
  for air traffic services outside its territory, is adequate, of
  required quality and timely. This shall include arrangements for the
  timely provision of required information/data to the aeronautical
  information service by each of the State services associated with
  aircraft operations.


and


  3.1.7 An aeronautical information service shall receive and/or 
  originate, collate or assemble, edit, format, publish/store and
  distribute aeronautical information/data concerning the entire
  territory of the State as well as areas in which the State is
  responsible for air traffic services outside its territory.
  Aeronautical information shall be published as an Integrated
  Aeronautical Information Package.


I was not able to find the decision to use AIP ENR 2.2 (other regulated airspace), but it's a matter of fact the information is found here, e.g. in the Norway AIP:


  5: Delegation of Responsibility for Provision of ATS  
  
  5.1: General Denmark, Norway, Sweden and the United Kingdom have
  arranged through the exchange of bilateral Letters of Agreement to
  transfer the responsibility for provision of ATS within parts of their
  own area of responsibility, to their neighbouring State.
  
  Procedures and communication within the said areas will be as if the
  airspace was an integral part of the FIR for which the nation
  mentioned is responsible.


Bidirectional scope

An effect of these articles is both the delegating and the providing states shall publish information about related areas. For example in the Spanish AIP, two cases are described.

Spain delegating to Portugal:


  By agreement between Sevilla ACC and Lisboa ACC, the air traffic
  service is provided by Lisboa ACC within the airspace defined by [...]


Spain receiving delegation from Portugal:


  By agreement between Lisboa ACC and Madrid ACC, the air traffic
  service is provided by Madrid ACC within the airspace defined by [...]