Yes, this term applies to philosophy. I think we would put Nietzsche in this category. Had he read Baudelaire? Probably, but it doesn't make any difference if he did or not, it was in the air, so to speak. Kant could be a candidate too, in the century prior, and not just Kant but the other early German idealists. Let's not forget Voltaire either, and maybe we could stretch a bit and even include Diederot as 18th century fin de siecle due to his late works. Now back to Nietzsche, we see him straddle the romantic and the modern. The key to the modern is that it stripped away all mediocrity and pretense. Picasso, Les Demoiselles d'avignon (1907), Schoenberg's early musical works: both shocking for the time, all pretense and fluff are torn away in these works. Had they read Nietzsche before they produced these works? Maybe Schoenberg had, but again it was in the air. And of course in science too around the turn of the century and afterward began to be extremely productive. So I think I think this concept is not just the "very end" of a century, as I'm sure you know. I should also note that it took a while for Nietzsche's books to be known, and for them to be translated and so on. And how could I forget Frege? Also, even though the neo-Kantian professors began to peter out, their students began to cut their teeth then and to forge new ways, so much was happening, and I'm sure if we looked back further we would find this happening with other philosophers in prior centuries. By the way, I take for granted fin de siecle means end of century, but it just so happens that new eras often emerge as things heat up toward the end of one century and they carry on into a new one.