Those that are delisted from bittrex and other exchagnes are basically not dead. They are either a security token or they do not meet the required daily volume for the exchange. Aside from that in severe case they are probably found out to be a fraud or scam coins.
They have their reasons why they are delisting it. I would say, better get rid of those tokens or coins as fast as you can because the likelihood of them dying is very high. Yes, small exchanges might accept them, but how sure are you that they can survive in the battle of crypto trading?
Because of the lack of interest of traders who made the exchange site delist some of the coins, some well-known exchange sites like bittrex have special standards for coins traded on their markets, to maintain their quality they do delisting. yeah it's better to sell when you have received a notification about delisting coins or stayaway of that coin if you dont want to get lose