My understanding from a GOV Employed MOT man (because that is all we have where we are) is that the vehicles are made and type approved and will be certified as such. To do as VW has done is an offence.
If you modify the vehicle yourself you are not committing an offence and cannot personally be prosecuted for such
If however an MOT man/woman/person notices that a part of an emissions control system has been disabled/modified/removed then the vehicle can fail MOT.
DPF removal does not usually involve removing the DPF as is suggested but actually involves removing the internals of the DPF which are a disaster waiting to happen anyhow as is an EGR
If the removal is well done the casing is still present and there is no noticeable
alteration of the system so no problem at MOT
EGR mod's are similar and there is little way in reality that an MOT man can say that either a DPF or EGR has been altered or for that matter most MOT men do not for the most part care.
I know the items are there to reduce various pollutants but they are far from reliable and often after a couple of years cause more problems than they ever solved.
The output from the later DPF equipped vehicles is now being argued by chest/health experts as being worse than any black soot ever caused (as seen on the TV News during the VW scandal for those who don't bother to research such things for themselves
The problem is the diesel engine/diesel fuel itself rather than the equipment or lack of equipment fitted.
So for me who has been blocking/removing/disabling EGRs for over 20 years mostly because they have stopped working or worse the crank bearings have seized due to oil that resembles grease rather than oil, I think they are and always have been a short term fix to a long term problem.
I have a similar opinion of DPFs. All they served to do was to allow a greater amount of exhaust gas recirculation without visible smoke which allowed reduced NOx via a cheap system
Today we expect our cars to run for years with minimal trouble. EGRs and DPFs don't fit in this minimal maintenance box and as to cost, well that is the biggest problem.
If all these things worked as they should we should not see smoke at all but we do and often.
So if I had a problem with either device I'd be right in there. Good remapper and end of problem.