It turns out the research for that doesn't meet evidence-based medicine standards. Almost all observational studies of human nutrition are junk science with small effect sizes and poor controls.

the evidence for that was mostly based on Ancel Keys' work around the 1950s and it has been pretty clearly shown that he cherry picked the data in his observational study. When analyzed properly, the link between any kind of heart disease and red meat disappears. Look up Gary Taubes work for thorough references

Source? You may actually be thinking of the lipid hypothesis (https://en.wikipedia.org/wiki/Lipid_hypothesis) that Ancel Keys developed.

As I gather, we still believe that red meat consumption leads to heart disease, but we're becoming less sure of his hypothesis that the elevated cholesterol resulting from saturated fat (found in red meat) causes heart disease.

There is an assumption here that there is one diet is the best in all respects. But I don't think that's the case. Diets have benefits and drawbacks. Like over the years we have adapted a little bit in this direction and then later a little bit in that direction and that's how we are where we are.