The terrible smog in big European cities is increasingly being blamed on the same diesel vehicles that, ironically, governments there have been promoting for the past several decades. I suspect more environmental restrictions will be placed on diesel vehicles in the future, and this very well might include increased exhaust testing during annual inspection requirements.
I would recommend anyone taking off their environmental equipment to keep all of it in the garage, including the ability to reprogram the reprogramming so that it can be put back to factory standards some day.
This is also needed for the owners of vehicles still in warranty in order to keep the warranties valid should something go wrong.