Remove vanished deletes the backups in our PBS that are no longer present in your on site PBS in the next sync job.
This can be useful for saving storage, but it also comes with risks.
If you or someone else accidentally deletes your backups on your PBS, they will also be deleted on our PBS.
This is also possible: if a hacker (or someone else who wants to do harm) deletes your backups, they will be removed on our PBS and there will be no backups at all.