I didn't know this was a thing!
It actually mentions 'corn' in Rings, but I believe corn used to be a general word for 'grain', not exclusively used for maize. So in that context it might just refer to wheat or something. I think the bigger issue is that there's never any mention of distillation in the books. Miruvor is called a 'cordial', which implies some kind of distillation, but that's about it. All other alcoholic beverages are fermented (beer, wine, etc.).
History doesn't provide many sure answers either, since distillation was pretty common in Europe by the middle ages, that being the historical basis for the setting of Middle-earth.
So, long story short, I wouldn't drink whisky in LOTRO just because it's not in the books.
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