@ToraToraTora
The table at the foot of that article, showing the emission standards 1 to 6 shows the extent to which output limits have been slashed by around 90% since the 90s. I don't have the statistics for the corresponding increase in diesel vehicles on our roads but, if I did, we could draw a conclusion on whether bulk quantities of NOx are up or down. That would enable me to say if demonisation was unfair or not.
As an alternative measure, one could look at petro company's combined sales figures for diesel, 1992 to present, normalise the data according to typical NOx output of available engine technology, assessed year-by-year and see whether our air is improving or worsening.
One aspect of that article which doesn't help answer your question is that it doesn't even touch on the issue of particulates and PM10s, which I always thought was everyone's chief concern about diesels.
Personally, I hadn't made the link between vehicular NOx and asthma conditions, which are famously on the increase amongst the under-25s, so thank you for drawing this to my attention.