Well, I checked. AFAICS, the headcode 1Z99 is still in use, but 2Z99 has been retired. That one was given to line-clearing trains returning from an incident or otherwise not involved in line-clearing. They are now listed as freights of an appropriate class.

>1Z99 was the highest priority in the UK railway system

I think it gets trumped by 1X01, unless that's the reason for the 1Z99.

Hopefully, 1X01 will never need any 1Z99 attention, except for maybe a plough ahead.