The reporting in the Tartan article is surprisingly poor. The current female enrollment across the campus is NOT “historically low” at 38.8%. I attended in the early 90s and confirmed my memory through a 1994 fact book and at that time female enrollment was around 30% and SCS enrollment was about 7% female.
That doesn’t help with your question, and my experience is far removed from whatever the current environment is. When I attended, many engineering and CS classes would have 1 or 2 women, and it was a challenge.
My college roommate’s daughter is currently a student, but in engineering, not CS. She doesn’t love CMU – it tends to be a quirky place socially and not always a typical college experience – but she is flourishing academically.
I suspect @Data10 is correct, that after the Supreme Court disallowed affirmative action in admissions, CMU’s 50/50 split for CS wasn’t sustainable. There may or may not be toxicity in the program, as you asked, but that’s unlikely the reason for the dip.