pleural thickening is not due to your smoking
however its not good for your health so stop.
Secondly pleural thickening can be a part and p[arcel of the previous infections you had and a sequel to it but it is benign in the way.
If you have a discomfort in the same region and the earlier xrays never showed a pleural thickening then it must be a small rim of pleural effusion
and needs to be investigated