When you rename it to .old it gets regenerated once you logon again and it seems to be triggered by opening apps according to ‘Modified’ date in file manager.
My guess: It tracks the history of used apps/ app history per activity.
After restoring the original one I had the old app history in Workspace behaviour > Activities > Privacy.
One thing for you to try is to clear history in activities settings before renaming it and check db/file size afterwards. You need to logout and logon again.