If you try to download any significant file with zero-upload, you will run out of peers that will share with you much earlier than you will download the file. It's not practical.

Most people that speak of leeching or not seeding really are talking about not seeding at all after they've completed. In fact, most clients will let you set upload speeds to a trickle but not zero (zero means unlimited in most clients). From a legal standpoint, that already means you uploaded.

It’s true that most clients do not support a zero upload configuration, but it’s not inherent to the protocol, and modified clients exist.

I’m not aware of any clients that will refuse to share data with clients that are configured to not upload. I don’t even see how they could determine that, especially in situations where there are no other peers to upload to, and given that stats are entirely self-reported and clients that send bogus numbers exist.

You would need a central tracker that cares, which is what private torrent communities rely on, but not public/DHT torrents such as those discussed here.

Seeders don't know how much data you shared with other leechers.