Currently, documentation for the extension.json schema is available via the source file and Manual:Extension.json/Schema. Per 1094382, the wiki page is outdated.

In the short term, we should update the wiki page to be accurate with the current schema. However, in the long term, we should look into generating human-readable documentation based on the schema file that can be published to doc.wikimedia.org.

See also:

• T376841: Render human-readable schemas on schema.wikimedia.org
• https://github.com/tomcollins/json-schema-static-docs
• https://github.com/coveooss/json-schema-for-humans