Basically, the runways are closed, marked with an X, runway and apron lighting deactivated, landing and departing of AI traffic is closed and finally, airport comms are disabled, to avoid communications from/to airport.
The option can be deactivated by unselecting the option 'ICAO Identifiers' on fsAerodata Config Tool.
When using a map tool like Little Navmap, they are displayed as closed airports, and the total of them can be listed using the Find function.
The changes are available starting with AIRAC cycle 1905 revision 1, and as any other navdata, will be updated following the AIRAC cycles.
Photos of EDDI Berlin-Tempelhof with the changes applied:
The runways are shown with an 'X' marking on the surface
Little Navmap displaying the closed airport; the list on the right shows the total of airports decommissioned; just filter by variable 'Closed' on menu.
Enjoy the flights!