For release notes per se — what changed from one release to the next — please see the following:

https://github.com/johnkerl/miller/tags

Note:

Head docs at http://johnkerl.org/miller-releases/miller-head/doc/index.html match head code at https://github.com/johnkerl/miller.
Release-specific docs at http://johnkerl.org/miller/doc/release-docs.html match release-specific code at https://github.com/johnkerl/miller/tags.

The following are full docsets as of the specified release:

Miller head
Miller 5.2.0
Miller 5.1.0
Miller 5.0.0
Miller 4.5.0
Miller 4.4.0
Miller 4.3.0
Miller 4.2.0
Miller 4.1.0
Miller 4.0.0
Miller 3.5.0
Miller 3.4.0
Miller 3.3.2
Miller 3.2.2
Miller 3.1.2
Miller 3.1.1
Miller 3.1.0
Miller 3.0.1
Miller 3.0.0
Miller 2.3.2
Miller 2.3.1
Miller 2.3.0
Miller 2.2.1
Miller 2.2.0
Miller 2.1.4
Miller 2.1.3
Miller 2.1.2
Miller 2.1.1
Miller 2.1.0
Miller 2.0.0
Miller 1.0.1
Miller 1.0.0