Seeds in fact do not have an "expiration date", rather it is a sell-by-day, just like food. The dates on seed packets are guidelines to help you figure out if your seeds are getting old, but it does not mean you have to throw them away if they are past their sell-by-date.
Every plant is different. Some, like lettuce, last longer, and others, like onion, go bad very quickly. Still, most seeds remain good for about 2 to 3 years.
Practice good seed saving techniques to extend the life of your seed.
Also, seeds from past growing seasons work great for growing microgreens, which are plant sprouts that have been allowed to develop their first leaves. If you are unsure if old seeds will grow, test some of them out as microgreens. Learn how to grow microgreens at this link: https://foodhero.org/microgreens-garden-tips-and-how-grow.
