In my opinion, the best place to implement the ability to rollback a schema change is in the database engine itself. In terms of how that would actually be impl

In my opinion, the best place to implement the ability to rollback a schema change is in the database engine itself. In terms of how that would actually be implemented that's outside my wheelhouse. I think the ability to have some _standard_ method of rolling back schema changes across all the popular database engines would be significant. Some engines implement some support for some things. Many DDL commands can be run in a transaction in SQL Server. I believe Oracle creates a savepoint of some kind after every commit and DDL commands typically imply a commit. But it varies across vendors and it's a lot to untangle. In practice a lot of rollback scripts tend to make the assumption that the change script succeeded without issue. When the change script fails for an unknown reason often the rollback script will fail as well, in the worst case also having partially succeeded.