It's implemented in the choking algorithm. Basically if bandwidth or upload slots are limited, your peer with preferentially upload to peers that have reciprocated to you. AFAIK most clients implement this.

Interesting, thanks!

It isn't so important now most people have fast enough internet.

Back when people left things downloading for days, there were severe shortages of people willing to upload, and tit-for-tat encouraged uploading, solving the issue.