whats a late game champion?

In older seasons matches used to have an additional phase called late game. It usually started after the match lasted at least 20 minutes (yes, in earlier seasons, games could last longer than 20 minutes). At that point, most champions were able to afford even 4-5 items!!! Like nowadays there are champions who synergize better with a particular item, in those ancestral times there were champions that used to work way better than others once they were in possession of at least 3 items. Now there are still late game champions, those who haven't been updated into this new era of LoL, and so still need at least 3 items, or have some other requisite for their damage that will come only later in the match (which will never come, since the late game no longer exists). ADCs used to be late game champions, but as you said, they've been updated. Nasus is a kind of late champion as well, he used to be actually scary. But he hasn't been updated, so he still needs a late game, hence the low win rate.

To be fair though, if you want to see something similar to a late game, with a chance of actually seeing a late game champion at work, you should try ARAMS. Those games can sometimes last more than 20 minutes, going as far as 30-35 minutes. Sounds crazy, but it happens.

Once the zeal is there, he is online.

Champions that are supposed to have a bad lane phase. Even though outside of Ezreal bot that doesn't happen.