If grass seed is kept cool and dry, it can last a long time. If it gets too hot or wet, it loses viability.

The rule of thumb is to keep the temperature (in F) and relative humidity below 100. So if your seeds were stored (for example) at 60F and the humidity was below 40%, they should be in pretty good shape.

They have found viable seeds that are hundreds of years old. The key is dark, dry and cool.

A quick test. Wet a paper towel and ring it out so it is damp. Put 10 seeds on it, fold it in half, and put in in a ziplock baggie. Put the baggie on the counter and check it daily. I assume your "deep shade" mix is mostly fine and creeping fescue. If so, you should see some germination in a week or so. Let it go a few days after the first seed germinates. Could the number that germinated out of 10 and you will get an estimate of how viable the seed is.

Refer to this article about Does grass seed go bad. I think the answer is about 2 years. As you mentioned, your seed was kept well during winter, so I think It will be ok to use in spring. May be it's depended on other factors.