You can perform introspection against the GraphQL API directly.
Alternatively, you can download the latest version of the public schema here:
schema.docs-enterprise.graphql (Enterprise Server 3.0)
This version of GitHub Enterprise was discontinued on 2022-02-16. No patch releases will be made, even for critical security issues. For better performance, improved security, and new features, upgrade to the latest version of GitHub Enterprise. For help with the upgrade, contact GitHub Enterprise support.
Public schema
Download the public schema for the GitHub GraphQL API.