See the r3 package's CHANGELOG.md.
R3 is distributed as a monorepo (poorly) mimicing the style of lerna. This was inspired by several other large open-source projects distributed this way, including React, Gatsby, etc.
This means that our CHANGELOG.md files are distributed alongside the package itself in the src directory.
For example, the package r3_portkit has a CHANGELOG.md in src/r3_portkit/CHANGELOG.md. If you'd like to check out others, check out the src directory in the root of the repo.