The value form parameter mentioned in the "SETDOWNLOADSTATUS" documentation highlights that the values that can be sent are: 'ONLINE', 'ARCHIVE' and 'RESTORING.
Requests made with these values fail due to the fact that TopCAT is using the DownloadStatus enum and not the Status enum. This could be because DownloadStatus is what is defined as the status in the Download class.
The documentation could be changed to reflect the types in DownloadStatus, possibly this specific line.