Second Update
The EU has now implemented the Commission Implementing Regulation (amendment) 2019/430 revising Article 4(8) to the EASA Aircrew Regulations, which was referred to by me a couple of months ago. This measn that the CAA will continue to endorse the Instrument Rating (Restricted) (IR(R)) rating on UK CAA issued EASA PPL(A), CPL(A), MPL(A) and ATPL(A) licences until 8 April 2021. The IR(R) may not be endorsed on Part-FCL licences issued by other EASA Member States.
Details of the Flight Test for the rating is included in Standards Document 25.
Those who want to read the revised Aircrew regualtion (not many, I suspect) can see the update here and the consolidated regulation here.