“Yeshivish is NOT a language. Its much less of a language than ebonics anyway. “
I heard a story about a tri-lingual Rosh Yeshiva whose student asked a question in shiur in yeshivish. Rather than answering the question, the RY requested that the student restate his question in his choice of Yiddish, English, or Hebrew, using proper grammar and vocabulary in either.
“unless Yiddish is your familiar language (which it still is for many). “
There is still one RY at YU who gives shiurim in Yiddish: Rabbi Gershon Yankelevitz. He is old enough to have known the Chofetz Chaim, and spent WW2 with the Mir Yeshiva in Shanghai.