As of July 2020, GitHub publishes its REST API definitions in OpenAPI 3.0 format here:

https://github.com/github/rest-api-description

Official announcement:

OpenAPI Description of REST API

GitHub published a full description of its REST API in OpenAPI 3.0 specification compliant documents.

The GitHub OpenAPI description contains more than 600 operations exposed in our API.
For visual exploration of the API, you can load the description as a Postman Collection.
Programmatically, the description can be used to generate mock servers, test suites, and bindings for languages not supported by Octokit.

The description is provided under two formats.

• The bundled version is preferred for most use cases as it makes use of OpenAPI components for reuse and readability.
• For tooling that has poor support for inline references to components, we also provide a fully dereferenced version.

Quarterly releases of the description are available for GitHub Enterprise Server and GitHub Private Instances, with versions like v2.21. More frequent updates to the description will be available for GitHub.com.

This feature is offered in beta as open source with an MIT license, [in this repository github/rest-api-description.

APIs.guru OpenAPI directory also contains unofficial definitions for GitHub REST API in OpenAPI 2.0 format, which may or may not be up-to-date:
https://api.apis.guru/v2/specs/github.com/v3/swagger.json (JSON)
https://api.apis.guru/v2/specs/github.com/v3/swagger.yaml (YAML)