No! Prometheus is and has been production ready for many years. Cortex is a clustered/horizontally scalable implemention of the Prometheus APIs, and Cortex has just gone production ready. Sorry for the confusion.
But readiness depends somewhat on your use case. If you're on a multi-tenanted cluster and you don't want to explicit trust your users / admins, how do you stop them from messing with your metrics whilst allowing them to maintain their own?
I typically did it via github flow, some others used the operator to give us many proms, some others would just suggest it's missing features.
Indeed, I could probably word my example better though. Apologies if I were putting words in your mouth.