A deprecated control is a control that has been retired for all technologies. You'll see the deprecated control icon when viewing deprecated controls in policies and in policy compliance reports.
When you edit a policy, you'll see a warning if the policy includes one or more deprecated controls.
Click the "Replace all now" link to replace all deprecated controls with replacement controls. Be sure to save the control. Each deprecated control will be replaced with one or more replacement controls for all relevant technologies in the policy. If a deprecated control had a user-customized value set in the policy, then the service will use the custom value for the replacement controls whenever possible.
If you don't want to replace all deprecated controls at once, you can replace some of them. Click the Deprecated Controls button, jump to a control and select the "Replace this control" link.