When you delete a package, the following happens:
After the package is deleted, its symbols continue to exist, but because they are no longer interned in a package they become eligible for collection at the next garbage collection. They survive only if there are useful references to them elsewhere in the application.
Note: Invoking the treeshaker has much the same effect on packages as deleting them. However, by deleting a package you regain some extra space taken up by hash tables.