When an SFTP user is deleted from the web admin portal, the SFTP user's files will still exist in Cloud Storage.
SFTP Gateway does not delete nor move (i.e. tombstone) the Cloud Storage path of the SFTP user's chroot directory. This is because SFTP Gateway leans heavily toward avoiding data loss situations.
If you need to clean up bucket objects after deleting the SFTP user, this needs to be done manually.
Also, bucket objects are not impacted when deleting a Folder from the web admin portal. Folders are just mappings or mount points. So if the SFTP Gateway Folder mapping is deleted, the underlying bucket objects will still remain.
In general, actions in the web admin portal do not delete files in Cloud Storage. File deletion in bucket occurs manually -- via the Google Cloud portal, or via an SFTP client like FileZilla where the SFTP user has read/write access.