It's only supported for the Legacy backup format right now. If you're using the New Backup Format feel free to upvote this feature here https://app.sleekplan.com/feedback/153430

sorry, gotta sign up for a "free trial" to upvote that feature with that link.

if you are using our Managed Backup you don't need to sign up for a trial. just login to your MSP360 Managed Backup account

Maybe a better question is how should I setup my backups for drives that will be rotating? will the "forever forward" work? Does the backup check the current backup media to determine what needs to be backed up?
Is there a way to "trick" the backup to work in a rotating hard drive situation?

Not at the moment, but it's on the roadmap

I guess I would ask why you or your customer feels the need to have rotating drives?
We backup our client's data files to two cloud locations each night (BackBlaze/Wasabi) and also do an Image backup each night to BackBlaze for Disaster Recovery. We keep roughly one week's worth of images in the cloud for each client.
We also backup the files and images to a large local external USB drive.
The cloud file/Image backups truly eliminate the need for rotating offsite USB drives. And those drives can (and will) fail at some point.
We do have one customer (out of 100) who wishes to keep their own "offsite" copy of the data so we have them bring the device into their office every 90 days and plug it, and we run manual file and Image backups to it, then they take the drive home.
It is of little real value other than to appease the "old-school" client who grew up using only local tape drives for backups.