Not in my house!
Just kidding. But seriously, an unopened bottle of Whiskey should last indefinitely, although there will be some oxidation occurring even in the unopened bottle. This will only have an effect over an extremely long period of time, but 20 years qualifies.
Once opened, the oxidation will accelerate slightly, but the good news is, it probably won’t ever “spoil” or go bad, due to the high alcohol content. And the oxidation takes a long time– think years rather than months– and even then, after a few years, it will just lose a little of its flavor and alcohol content. It will still be safe to drink, and any difference in flavor would be barely perceptible.
But the real point here is that once it’s opened, why on earth wouldn’t you just drink it? Don’t feel like you have to drink it all in a few months, but definitely don’t wait 20 years.
I once found an unopened bottle of Canadian Club in my in-law’s house that they got as a wedding gift. It still had the government seal on it, dated 1967. Assuming the stamp went on the year it was bottled, it means this whiskey, which is a minimum of 6 years old at time of bottling, was distilled no later than 1961… during the Kennedy administration. I naturally opened it and drank some. It was exceptionally good for a Canadian whiskey. Interestingly, it tasted nothing like the current run of Canadian Club; it was much richer and fuller in flavor, and it was even a darker color.