if its true that your lungs return to normal after a year or so of giving up smoking, why then if you need a chest xray do they write on the notes ex smoker, obviously then your lungs are not back to normal as in a non smoker.
Lung cancer can have a latency period of up to 30 years. Your notes are being marked so that the doctor analysing your xray will ensure they look for any abnormality.
It's good they do that. My dad gave up smoking at 40 and died of lung cancer at 60. At least if it's flagged up in your notes that you are an ex-smoker they might take extra care to look for the warning signs of lung cancer.