My local pet shop does offers where out of date cat biscuits are half price.
I bought her some meowing heads 400gram bag to see if she liked it and she wolfed it down. It's very good quality food.
I went today to buy hopefully a bigger bag as there was one a month ago but I didn't think it would still be there being half price. But it was.
Meant I paid £9.50 for a 2kg bag of meowing heads. Usual price is £18.99.
Made me realise that not everyone would buy out of date cat biscuits but I have no issue as long as the bag is sealed and has no rips or tears.
The way I see it is, cat biscuits are a dry food anyway so unlikely to go off very quickly and the vitamins take a little while to degrade so it's ok.
Wouldn't do it with wet food though