Even the difficult captchas are getting easier and easier to solve all the time by bots as computers and AI technology gets better.
Traditional captchas were always a stop gap measure because computers are specifically designed to solve problems like this and traditional captchas mostly just relied on computers not being good enough to solve them in a reasonable amount of time, but they’ve been good enough for a while now.
This is why Google’s latest iteration of their captcha is moving towards not actually using “solvable” things anymore. It uses machine learning to determine if the information put into a form on a website is legitimate or not.
Which is the way to combat bots in video games as well. When you have all the data about how a player is playing the game(which Blizzard collects a lot of that), then it’s pretty easy to find patterns to see which ones are bots and which ones aren’t.
At that point it’s just a matter of having your own AI go through the data and flag suspicious accounts for review by a real person.