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