The phrase 'remain outside controlled airspace' has always been an option for ATCO's to use, however, it recently became mandatory if the ATCO cannot give a joining clearance etc to the pilot without delay. Now, i have to apologise here, i can't remember which MATS update/ATSIN etc etc it was in so i stand to be corrected.However, i'm fairly sure that's why the phrase is more 'prominent' now on the r/t.
It was CAP493 SI 2008/02, which has been withdrawn. It was a reminder of the importance of the phraseology which already existed.
CAP 493 SI 2008/02: Use of Phraseology 'Remain Outside Controlled Airspace' | Publications | CAA