ESP8266 Arduino Json object array deserialisation into struct
No memory is allocated for the strings.
You should declare your struct like so:
struct Departure {
   char destination[32];
   char departure[32];
   int  deviation;
} depart; 

This assumes the max length for both strings is 31 characters.
Then, pass the max size to strlcpy():
strlcpy(dep_arr[i].destination, ResponseData_Metros[i][&quot;Destination&quot;], 32);
strlcpy(dep_arr[i].destination, ResponseData_Metros[i][&quot;ExpectedDateTime&quot;], 32);

If your strings are very long, you can use a String instead of char[].
See the example JsonConfigFile.ino, it shows all of this.