AFAIK think it's like spec in this respect. There are hacks around it but it's not supported, https://github.com/metosin/malli/blob/3599fbd7fe1eba96c35e5c39073397cba0984b6a/src/malli/core.cljc#L975-L982 is concerning presence, not absence.

another hack I remember from spec is using map-of's keys with a restricted spec for keys.

when I https://github.com/typedclojure/typedclojure/blob/bb973bce811c111a814aad4889aa2a083a5e761d/typed/malli/src/typed/malli/parse_type.cljc#L148-L151 a Typed Clojure => malli translation, the best I could find was {:closed true} to prevent all other keys.

That might be what you want actually.

Unfortunately not, my use case is a data model like GeoJSON where it's open by default but some keys are reserved and cannot be used in certain contexts

You can require a map be closed in malli