I think (Nathan can add to this if I am incorrect) that there just isn’t much need for one. Nothing else references it, the table probably rarely grows beyond 100,000 rows (and most times far fewer than that), and creating a unique constraint on it in a database-independent way would make the code far more complex with little benefit. I think one of the design goals was for the Liquibase table to have as little impact on the database as possible.
Principal Software Engineer
Datical, Inc. http://www.datical.com/