I have created several observation lists, added objects, linked them to observations, later removed the objects, and tried to delete the observation list, but was told that the list could not be deleted because there are observations that reference the list.
In some cases I had unlinked all observations before removing the objects from the list. After updating to v. 6.0.4 and waiting for the lists and observations to synchronize, these lists could be deleted.
However, if objects were removed from a list before all observation references to the list were removed, the list cannot be deleted.