But look at the graph on page 2 of the government's vehicle excise duty evasion statistics and you can't miss the rise from 2014 :There you go then, replacement methods of detection and enforcement have proven to be more efficient than traffic wardens spotting the wrong coloured disc. As wardens tend to operate within 'town areas' ANPR operators are active on all streets and have a better detection rate.
And on 22 November 2019 the Times reported that "The proportion of drivers evading road tax has almost tripled since the paper disc was abolished, blowing a £94 million-hole in the public finances. The owners of 1.6 per cent of vehicles tracked on UK roads this year failed to pay vehicle excise duty, equivalent to 634,000 cars, vans and lorries. Before the system was digitised in October 2014 the figure was only 0.6 per cent".
It isn't though a case of either traffic wardens, who I don't think spent much time scrutinising tax discs, or ANPR operators which have been used since 1979, 35 years before the discs were abandoned. Both the disc as a visual indication of tax paid and ANPR should have a role.