There have been no released statistics showing any such thing… it is just CC rumor and gossip that gets passed on from thread to thread. Also, nearly 50% of Harvard students are in STEM… it is not just a recent phenomenon… it has always had a relatively high % of STEM. In fact, Harvard has more students in physical, biological sciences and math, by far, than Stanford. Stanford, obviously, crushes Harvard in number of students in engineering disciplines.
From the CDS 2016-2017:
Computer Science: 06% at Harvard, 14% at Stanford
Engineering: 04% at Harvard, 23% at Stanford
Biological Sci: 14% at Harvard, 06% at Stanford
Math/Stats: 11% at Harvard, 03% at Stanford
Physical Sci: 07% at Harvard, 03% at Stanford
Psychology: 06% at Harvard, 03% at Stanford
Total: 48% at Harvard, 52% at Stanford
Note: 17% of students at Stanford are reported as primarily “Interdisciplinary Studies”, a significant fraction of that could also be specifically in STEM; hard to say from the CDS alone.