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Spec has numerous uses. We use it heavily in production for data validation/conformance: https://corfield.org/blog/2019/09/13/using-spec/

I bet you are thinking of https://clojure.org/guides/spec#_instrumentation_and_testing - it was always just one use of spec among many.

We don't have instrumentation in production, ie. we spec much more at dev time. But for things you'd validate anyway spec is a good match and probably about as performant as hand-written validation.

The only strange use of spec I mentioned is validating db queries. For us updates and deletes are rare enough that perf doesn't matter.

No. It was always designed to be configurable so you can keep on the API endpoints, but deactivate them internally.