Can we use PCB(immersion gold) after its expiry date? if yes, any process to be done to the PCB to extend shelf life & what could be the extended shelf life?

 

I am guessing that the question is asking about the immersion gold plating chemical mix, relative to it functioning as intended. Usually, storing chemicals in a cooler environment reduces any decomposition processes. And ALWAYS, properly closing the container extends the product life.
Probably the plating mixture, if it has not separated or settled, will work as intended but more slowly than when it is fresh. My experience with other chemical processes has been that they do not react as rapidly after they age. In some cases I have had to raise the temperature a bit for the reaction to occur. This was both with gold plating and with hard epoxy setting. In both cases the product was several years old.