The releases section in github[1] does a fairly good job at listing the changes. There is also the Jira release section[2]. Not sure if these are comprehensive.
[1] https://github.com/liquibase/liquibase/releases
[2] https://liquibase.jira.com/projects/CORE?selectedItem=com.atlassian.jira.jira-projects-plugin:release-page&status=released