Where did the practice start of answering questions here with simply a jumble of letters?

pointgamers have been doing it since forever, but the SPAMBOT-attack pretty much started december 28, 2018 (your timezone may vary)

You'll see literally DOZENS of posts in this category (Yahoo Answers) about the spam bot attack that began on Dec 28th. They aren't humans trolling, they're automated bots (computer programs) that appear to be trying to take down the site. Report as many as you are willing (and in the future you can save 5 pts by scanning the YA category before posting about an issue, to see if others have already done so).

Not a "practice", It is a return of the bots that soon will flood the questions with spam once they get enough points from "answers" like crggws. I thought at first it was just kids with new computers making many accounts so they could thumbs-up themselves (given that I first noticed it after Christmas) but it seems too deep to be just that.

I think that people are doing that to get more points in Yahoo Answers. Also, they may be unemployed and/or bored.

The practice originated in ancient china

The practice originated in ancient china