I heard of a very intensive and long term study, about 10-15 years ago, that showed that stretching before exercise, has no benefit whatsoever, and actually can be detrimental to muscle health.
I believe it was ignored for it embarrassed to many phys. ed. trainers and professionals.
But all guide lines on anything seem to oscillate every few years. The knowledge does not grow.....it just switches back and forth. Like the benefits of coffee. Some years it's good and some years it's bad.