I've never heard of anyone using it in production, period. The one thing FastAPI had going for it back when it was announced was a list of companies and groups with it deployed that minute.
Is it a "1.0 because we think it's done and you should try using it now. Maybe it'll work." Or "This is a solid 1.0 that has been tested in production. You can expect reasonable api stability."
FastAPI launched with option three: "We're going to get our friends to use it in production so we can say it's deployed by Microsoft, but, this is definitely a pre-alpha."