How does "Ode to Autumn" by John Keats relate to the 21st century?

This is a great question, especially since it requires some careful deliberation and cannot be answered immediately. How one answers this question depends on how one relates to the 21st century, and what associations that term suggests for each individual. For my part, I believe that John Keats' "Ode to Autumn" reminds us that the harvest, and therefore our very livelihood, relies upon seasonal patterns. 

In the 21st century, it's easy to forget that food must be grown and harvested. When we're hungry, we can walk into any grocery store and enjoy nearly limitless options. We can buy pre-sliced bread and avoid the arduous task of baking it ourselves, buy exotic fruits imported from thousands of miles away, and buy any vegetable at any time of the year, no matter the season. We live in an age of instant gratification, one that does not require us to consider the time and labor needed to produce food. 

Keats' poem reminds us of reality. In "Autumn," he employs many harvest images, such as "bend with apples the moss'd cottage-trees" (5), "fill all fruit with ripeness to the core" (6), "the winnowing wind" (15), and "half reap'd furrow" (16). He combines vegetable imagery with phrases such as "winnowing" and "reap'd," both of which are harvesting terms that remind us that the process of acquiring food requires strenuous physical exertion. Additionally, in the first stanza of the poem, autumn is described as conspiring with the sun, and by extension with summer, to bring about this explosion of food.

In short, Keats reminds us of two things: first, it takes time to produce food; produce must be planted months ahead of time, as it takes a whole summer to ensure that most vegetables can be harvested in the autumn. Additionally, harvesting requires extreme physical exertion. While the process of harvesting is much easier now than it was in Keats' day, it's still important to recognize that instant gratification is not realistic when it comes to food; everything we eat (i.e., what we use to survive) relies upon extensive, cyclical, often arduous natural rhythms. It will be useful to remember this fact the next time you visit your local grocery store.