How does "The Hollow Men" by T. S. Eliot reflect the reality of humanity in the early twentieth century, and is the meaning of the poem still applicable to the twenty-first century?

"The Hollow Men" emerged out of the aftermath of World War I. It can be seen as a paradigm example of modernism in that it reflects a cultural environment in which all the old certainties have been destroyed. Deprived of all the old moral, aesthetic, and political certainties by this cataclysmic conflict, Western humanity has been hollowed out, no longer able to draw on centuries of cultural achievement for its values and principles.
The contemporary relevance of the poem lies in the fact that, in the current climate, many of the certainties that formed the bedrock of Western civilization during the Cold War appear to be under serious threat. In this era of fake news and populism, the values of liberal democracy, the rule of law, and internationalism are under sustained assault from a variety of sources. Once again, Western humanity finds itself hollowed out, deprived of all the old certainties by massive upheaval (in this case the 2008 financial crisis and its aftermath, whose damaging consequences are still being felt to this day).

"The Hollow Men" is largely a poem of despair, decrying the emptiness felt by many in the early twentieth century following the terrible destruction of World War I and as fascism and state communism began to rise. Eliot repeatedly compares men to scarecrows, suggesting a disconnect from the heroic way in which they might try to frame themselves and the way they actually are.
We can understand the modern era in terms of people reacting against the terrors of war, industrial society, and the failed promises of the political systems that dominated the world. In contrast, post-modernism is often discussed as people adjusting to this. For example, there is a general understanding that politicians and corporations are corrupt but little expectation of truth or purity.
In this framework, we can see Eliot's poem as a modern lament for the hollowness of the world and these themes as setting the stage for the world we live in now. I would say "The Hollow Men" very much applies to the present day, but that it's meaning and tone will read differently in the different context we live in. I think few would really argue that world systems live up to the ideals they espouse, so i think framing the world and its leaders as hollow is still a compelling depiction.

"The Hollow Men" reflects a society that is filled with meaninglessness and symbols that have ceased to signify anything. One of the two epigraphs of the poem refers to the death of Kurtz in Conrad's Heart of Darkness. Kurtz is a revered figure in Conrad's novel until the main character, Marlow, finds out that Kurtz is a fraud; Kurtz therefore stands for the meaninglessness of life and of its spiritual deadness.
The beginning of the 20th century was marked by the loss of belief in established ideas and by widespread disillusionment, particularly after the death and destruction caused by World War I. Countries were divided by a belief in western-style democracies, a belief in Fascism, and a belief in Communism, but all systems resulted in war and in destruction. In the face of this wide scale destruction, it was hard for people to sustain their faith and hope.
Eliot reflects people's disillusionment with the hollow men in his poem. These men have a "headpiece stuffed with straw"--in other words, their heads are stuffed with nothing but straw. These men are so hollow, so desiccated in their ideas, that they seem to not be able to enter "death's dream kingdom." Their life does not end with a traditional death; instead, it seems as if death characterizes their hollow lives as they exist in a barren "cactus land." The cactus is a symbol of their spiritual desiccation. Their lives are drained of meaning; they are dry and without sustenance. Instead, they wander as if in a "valley of dying stars." They have no stars to guide their journeys but instead live without guidance and without hope.
Some might argue that the meaning of the poem still applies today, though you have to figure out your own take. Is today's world filled with meaninglessness and with "hollow men" without substance, or do people today find some sort of meaning in their lives and some kinds of hope?