Reddit reviews Knott's Handbook for Vegetable Growers

Yeah, the trouble with that data, widely disseminated on the internet, is that it originated from a handbook for commercial growers, who don't have time and money to waste on planting 20 acres of tomatoes for Heinz that aren't going to germinate reliably.

So yes, if your livelihood is on the line, you should buy fresh seed stocks regularly. But for every Florida tomato mogul out there who's buying new seeds every 4 years, there are probably hundreds of home gardeners, and especially seed savers, who are still planting seeds from 5 to 10 years ago, or even older. There are people here in the subreddit who have found seeds in their grandpa's effects from the 1960s that they planted, and which grew.

So like everything else on the Internet....it depends. Seeds kept cool and dry can last a long, long time. I still have tomato seeds from 2012 that I used in 2016, and they came up fine.

Knott's 4th edition, 1997.
http://www.ipm.iastate.edu/ipm/hortnews/1999/4-2-1999/veggielife.html

https://www.amazon.com/Knotts-Handbook-Vegetable-Growers-Maynard/dp/0471131512