While "The Wild Swans at Coole" doesn't seem to be about death in any obvious way, there are plenty of words and references that make us think of death and endings. The speaker imagines the swans leaving at the end of the poem (an act of abandonment that can be seen as a form of death) and he also speaks of a certain period of his life that is over—dead and gone, buh-bye now. On the biographical and historical tips, it is difficult not to think of World War I while reading this poem, a war that claimed tens of millions of lives (including Yeats' friend Major Robert Gregory) and destroyed Europe's innocence.
The swans are merely a distraction, nothing more. They're just something for the speaker to look at so that he doesn't have to think about the inevitability of death. Good luck with that, buddy.
The speaker emphasizes things associated with endings—like twilight (end of the day) and autumn (end of the year)—so he can avoid talking about his own death and the death of others (like Major Robert Gregory).