Your existing CoW work stays on your device
Leaving Fleet does not remove the CoW data already on your device. Your Caves, Chests, bookmarks, user settings, password protection, and hidden-content settings remain there, so you do not lose the work you have already done.
Only the Fleet cloud copy is deleted
When you cancel or leave CoW, the cloud data used for Fleet sync is deleted within 30 days. This is done for privacy: CoW does not keep a cloud copy of your data or use it to continue tracking you after you leave.