So, it's incomplete. Even the "Poetical Works" of Yeats reads 1920, so that is incomplete.
I did check again and find that they reissued the Collected Works. And according to Life+70, his works should be Public Domain. Though not entirely sure of the revised editions.
But well, we might have to find the other sources. Here the Bibliography on wikipedia in any case:
https://en.wikipedia.org/wiki/W._B._Yeats_bibliography
EDIT: Checked up
wikisource, which states that under
US Law any work before 1923 is in Public Domain, but all of his work is in the Public Domain in countries and areas where the copyright term is the
author's life plus 70 years or less. So, the US has to wait a while for PD works of Yeats, that also means that not all works of his can't be uploaded to MR, for now.