I pinged the author - this is the json-schema package to use:
https://package.elm-lang.org/packages/json-tools/json-schema/latest/
Its a more up to date fork of:
https://package.elm-lang.org/packages/1602/json-schema/latest/
presumably to publish under the json-tools name.
Seems like this is good replacement for NoRedInk/json-elm-schema and provides decoding and encoding of json-schema, as well as a builder DSL for constructing them. So that is a nice starting point for OpenAPI 3.0.