Taste aside, is it safe for drinking? Whiskey that was left inside a stainless steel flask?
Whiskey is quite high in alcohol, on the order of 40% by volume, and is not hospitable to pathogens growing.
The flask is intended to hold liqueur, and so is made from or lined with a food safe material, such as food grade stainless steel (assuming you have one from a reputable manufacturer).
So yes, it should be fine.
Remember: when it was distilled, the whiskey was probably held at different stages for long periods in a stainless steel vat.
My stainless flasks, USA made, purchased from distilleries indicate that spirit alcohol (of any proof) should not be stored for more than 3 days. I've never pursued an explanation for the statement. The flasks generally don't have any contents left by the beginning of the third day. While this may not constitute an answer, it's worth considering all the same.