I am using Devonthink 3.5.1 and still have a Mobile Sync Group in one of my databases. Are these groups still used? Can I delete the group and its contents without deleting the items from the database itself?
No. The Mobile Sync group has been out of use for quite some time now.
You can delete it but if you don’t have replicants or duplicated in it, you would be deleting the files.