It is and was forbidden, but there are exceptions for example if the philosophy is being used against Jews/Judaism then it needs to be learnt to be able to save Jews/Judaism, but in our generation Jews/Judaism are left alone and respected, there is no need to learn it
Source Shulchan aruch harav hilchos Talmud Torah 3.7 (his sources you can see in the link)
Partial translated
The mitzvah of learning Torah (day and night) means you should not be learning other things, but sometimes and a little bit it is permitted for a talmid chachom to learn other knowledge, since he can learn from them Torah and fear of G-d but not other people (only a talmid chochom)
This is only if they are not books of minim which are books of philosophers of the nations of the world which were minim and kofrim of "G-d's care of the world" and of prophecy, since it is forbidden to read and(or) look in them always even to learn from them some musar and fear of G-D, so even when their words are brought in Jewish books, you need to be careful (to stay away) from them, and about them our sages said "the one that reads outside books does not have a portion in the world to come, the only reason some sages delt with them was to answer them and to strengthen our religion, and in those times the time needed it, to answer the minim of the nations of the world, that where in those generations argument with the Jews, but not in out generations.
PS it is possible that some Jewish sages know it but it does not mean it was permitted for them to learn, even if you ask them they might confess that it was wrong