I need to disable 'Sync' on a Hosted Feature Service that contains 2 layers BUT the check box is grayed out.
I find that I CAN disable/enable Sync on other services I have authored that only contain 1 layer.
I usually temporarily disable sync when I'm overwriting a service as replicas are removed, allowing subsequent overwrite. The other method of replica removal (from the REST end point URL - un-registering replicas one by one) is tedious as this service is widely consumed for offline Collector data creation an many replicas, created.
See screenshot below for the 2 layer feature service with the grayed out 'Sync' control:
The solution to this comes from a wonderful ESRI solutions engineer, Krithica Kantharaj: "If you have a “View Layer” created from a feature layer and have sync enabled in the “view Layer”, then the parent layer’s sync will be disabled. Check if you have any views created from your hosted feature layer (and check the view layers sync settings)."