I currently have 19 opened partial bottles of whisky in my single malt "library" that has been in existance for over 20 years. They are kept in the dark with humidity around 35%.
I fell in lust with about 6 of them and I use them up and replace them. Recently I decided to revisit the orphans.

Found out how to "unglue the cork" after having to filter out cork bits and replace the cork. Invert the bottle to soak the cork and let it sit for a few minutes. Suspect the essences that escaped are part of the "glue".

The lower the level in the bottle when "forgotten" the worse they fared. These are at least drinkable with a touch of distilled (Reverse osmosis) water. The alcohol seems to have stayed and the water and some essences of flavor have escaped via the cork. Oily notes are still there, but the subtle flowery and heathery notes are gone. Can't say anything about smoky notes as these get used and replaced.

Any ideas are welcome. Some like the Cadenhead's - Cherry wood matured, cask strength Springbank 8 (dist 1985/bottled 1994 ) were great and not cheap ( well over $100 ).