Maybe the coin has violated the rules set by each Exchange. So that the coin was expelled from the Exchange and seemed to have been lost. You must be able to ensure that the coin is still active. By checking through their website.
This could be true, and is surely the case for some of the coins that were delisted,,, but let me tell you, the main reason is because the coin has stopped paying the fee to the exchange for allowing them to list. Not all exchanges practiced this but it was a big scandal made to come to light last year that project developers or coin owners were asked to pay a "listing fee" to exchanges to list their coin for a certain number of months.
I dont think that all exchanges are charging the coin listing to the developers. Only the reputed and big exchanges are charging to list your coins balance other coins are listing freely until you dont have fully working projects and network is working fine.
They are only delisting the coins which dont have their network working and who dont have enough trading to cover the cost of coin maintenance.