There's some prior art here from Clojure, where defn- creates private definitions and defn public ones:

https://clojuredocs.org/clojure.core/defn-

In Clojure this isn't syntax per-se: defn- and defn are both normal identifiers and are defined in the standard library, but, still, I think it's useful precedent for helping us understand how other people have thought about the minus character.