If a device which data (feed/actuator/alarm) you have put in a semantic group has been deleted/removed, the data’s name label will turn grey with No device notice displayed on it. A similar type of notice will be displayed in the semantic group widget as well.
You can remap the feed/actuator with another one, but it has to have the same reading type (e.g. temperature to temperature only), and needs to be expressed in the same unit (temperature in ⁰C, not F) as the previously removed feed/actuator/alarm. This way, removed device history will be remapped to a new device.