I quote from the paper issued by the Ministry of Agriculture and Fisheries in the UK <<It was found in the late 1970s that NDMA can be formed during the kilning of malt. As a result, changes to the malting process were introduced by brewers to reduce the formation of NDMA>> Originally only beer was supect but after furthern reseach in the 1980's it was discovered that whisky also contained this chemical.
The whisky industry looked at various ways to combat the production of NDMA and this was one way that was considered but never actioned, as far as I know. I would say that all Scotch whisky distillers who malt barley on site made a slight alteration to their process as did all commencial malters which means that all whisky produced since the mid/late1980's have either a zero on exceedingly minor inclusion. This was checked again by the UK governmnt in the early 90's and it was discovered that there is, or possibly was, more of this specific chemical in well done toast or smoked sausages that in any whisky samples checked from the late 1960's.
So carry on enjoying your Scotch but be careful how dark you make your toast