A vehicle will only fail if the CAT is missing if it is subject to a full CAT test, therefor Diesels are fine! 
Going forward, my interpretation of this is that it will not be fine.
If a cat/DPF was fitted from new, it's got to be there.
Need to clarify this as to how far back it goes, as I would ideally dump the cat off my 1996 Golf3 TDI, but don't want a PITA come MOT in August.