See the Gatsby CHANGELOG.md
Gatsby is distributed as a monorepo using lerna. Several other large open-source projects are distributed this way, including babel, react, etc.
This means that our CHANGELOG.md files are distributed alongside the package itself in the packages directory.
For example, the package gatsby has a CHANGELOG.md in packages/gatsby/CHANGELOG.md. If you'd like to check out others, check out the packages directory in the root of the repo.