Why is there a folder called .stversions inside my Platform Space?

.stverions is used to help save deleted or updated files in case of accidental changes

.stversions is the name of the folder used by SyncThing to save prior versions or copies of deleted files if a form of versioning is used in a Platform Sync.  When versioning is turned on, copies of files that are changed or deleted are created in this folder. Based on the settings, a certain number of versions of these files are kept for a certain number of days. Once the number of days or the number of versions has reached its limits, then the older files are deleted to prevent filling up the drive. 

This process is to ensure recovery of accidentally deleted or changed files.  This folder acts like a recycling bin to be able to recover these past versions of 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 a storage space reaches 99% full, syncs will not run.