YIVO was never “official”. It was a “wannabee” that few Yiddish speakers took seriously (perhaps due the fact they tried to create a standard dialect of Yiddish based on vowels from one dialect, consonants from another, and grammer from a third). YIVO’s version of Yiddish is useful in some universities, solely for the purpose of teaching people who have no desire to communicate with native speakers. Indeed, YIVO Yiddish is similar to Esperanto. How useful a language academy is can be debated (note the difficulty of the Hebrew and French academies in getting people to refrain from adopting English terms).
Living languages constantly evolve. Note how English has lost its 2nd person singular (thou), its subjunctive (“I be”, “if I were”) not to mention its neuter gender. Living languages absorb words from other languages. Due to extreme traumas (e.g. the holocaust, and the mass migrations of the last 150 years), Yiddish has changed radically. Also note that many native speakers of English have trouble with literature produced a few centuries ago (to most Americans, Jane Austen and the Declaration of Independence seem quaint, and Shakespeare is almost incomprehensible). There is no such think as a “pure” language, and the only unchanging languages are dead ones.