.stversions is from Trashcan Version being turned on with a Sync Share. When this is turned on, there is a set amount of days that when files/folders are deleted within a Sync Share, they are placed into the .stversions directory for a period of time until they are permanently deleted. This is to ensure recovery of accidentally deleted files. This folder acts like a recycling bin to be able to recover files. If files are confirmed that they are no longer needed within these directories, these may be deleted and will need to be deleted on all devices to create space. If storage is full, syncs will not run.