This is a very interesting question that causes us to look beyond the historical context in which this tremendous treatise was based and see if it still has any relevance for us today. One of the marks of universal literature is the way in which it continues to speak to people in a variety of different time periods and contexts, and the fact that this essay is still studied just as much today as it was then clearly points towards its inestimable value.
This essay's historical value is characterised in the satire that it contains. Swift attempts to show the British how inhumane they are being by ignoring the situation of the Irish famine by presenting a deliberately monstrous "solution" to solve this social issue. The abhorrent idea that he presents was meant to act as a mirror to the British so that in this idea they could see too their own abhorrence and involvement in not doing anything to appease the situation.
However, this essay has relevance to any human disasters resulting from bereaucratic incompetence around the world. Consider the issue of global warming and the lamentable lack of interest that the USA showed in the Kyoto agreement of 1997. This would be an excellent topic for an updated "Modest Proposal." Situations such as this show that we still have a lot to learn from Swift's original essay, and it is still immensely valuable to us for this reason.