Do you mean they were once fresh fishmonger sardines with a sell-by date that has been exceeded because the fish have since been frozen? You don't often see ready frozen sardines. The official advice for fresh meat/ fish is to use within three months of home freezing tho most people exceed that since freezing prevents bacteria growing.
Cats digestive systems are not like human ones as they are far more acidic and far shorter - ideally suited to eating RAW meat and all the risks that entails. As long as the fish looks and smells fine I wouldn't be too bothered, kitty will tell you if his sensitive nose detects anything you don't. I 'd be more conservative with prawns as they have a higher risk of hosting nasty bacteria.
Personally I'd defrost the fish in the fridge and steam them in the microwave immediately before serving (glass or china dish, small amount of water, cling film - break up/ turn fish every 30 secs). If you ensure they are cooked right through - meat should change colour - almost all microbes will be deaded.
