Why does the JSON schema for 3.0 specify patternProperties for component names but allows additional properties?

[handrews]

Discussed in #2439

^{Originally posted by ryankinderman January 11, 2021}
I've been working on some functionality that uses the JSON schema to help validate OpenAPI schemas, and recently noticed that the 3.0 schema allows any value for {name} in /components/{component}/{name}, while the specification states that "All the fixed fields declared above are objects that MUST use keys that match the regular expression: ^[a-zA-Z0-9\.\-_]+$." It seems like this could have been validated automatically by simply adding additionalProperties: false to the JSON schema.

My question is: Was not including additionalProperties: false in the JSON schema for 3.0 just an accidental omission, or was there some rationale behind not doing it?

Thanks for clarifying!