While looking for a human-readable schema, I found this text:
Schemas represent a new type of MediaWiki content, JSON schema objects, which adhere to the JSON schema specification, version 3.
I don't think this is correct. The schema defining what is a valid eventlogging schema is itself written as a full JSON-schema: https://phabricator.wikimedia.org/diffusion/EEVL/browse/master/schemas/schemaschema.json However, it restricts eventlogging schemas to something which is not JSON-schema, nor even a subset. We need to clarify this for developers, and point to the actual schema or describe it in words.