Ina FriedIllustration: Brendan Lynch/AxiosThe percentage of workers in the U.S. who say they are using AI at work has remained largely flat over the last three months, according to a new study commissioned by Slack.Why it matters: If AI's rapid adoption curve slows or flattens, a lot of very rosy assumptions about the technology — and very high market valuations tied to them — could change.Driving the news: Slack said its most recent survey found 33% of U.S. workers say they are using AI at work...