Hmm. Thank you for that, I seem to be mistaken there. I wonder if that was added after I acquired my knowledge of JSONSchema? Or if it was some weird thing w/ Swagger where it wasn't supported there.
Swagger (OpenAPI 2) and OpenAPI 3 never supported JSON Schema. They support a subset of the draft 4 with custom additions. OpenAPI 3.1 added support for additional schemas where you can use whichever draft you want.
Might just be confusion with swagger / openapi, which is a complete mess of procedural constraints reminiscent of XML Schemas.