It appears to be one of the features that has not made it across yet, there are a number of them.
It may be added to v7 later?
PS.
You do not need to wipe free space on an SSD because the TRIM command already marks deleted files as being free space, making the data inaccessible without significantly shortening the drive’s lifespan.
The SSD controller’s Garbage Collection and Wear Leveling then complete the job.
Wiping free space simply uses unnecessary write cycles on an SSD.