To update you need the data so you can rehash it. A typical tracker only has the torrent files and would take a very long time and spend a lot of possibly expensive bandwidth to download all the tracked torrents. And some torrents may not be seeded all the time.
If nobody has the data, the torrent is effectively dead and most private trackers will remove it in time. If it's not dead, they can incentivize seeders to re-hash/re-upload a v2 compatible torrent if they really wanted to.
Private trackers tend to gamify quite a few aspects of their sites (obvious example being the seed ratio itself).
They often just have the magnet links. I haven't looked into how that works, but I think the complete data is only on the seeders' machines, and the torrent propagates through DHT.
