They delisted those coins because of how low the volume is being traded on that day. Also it's not that worth to actually trade those coins lol nowadays so that shit is fine.
It is necessary to invest money only in those old coins that are still worth something.
It is, but most of them are not and it's dying anyway so it's completely useless to invest.
So, try investing in coins that are now alive and capable of fighting to survive in the future.
You can't just try and blindly pick them as they are sharing the same potential and could survive in the future especially today.