If you are using acceptance rates as a proxy for difficulty of schools in a given year, you need to adjust by desirability of location and perceived seriousness of degree programs.

And across time you have to adjust acceptance rates for the fact they are all way lower now because kids send out 2-3x as many apps.

A lot of kids even 20 years ago were applying to schools in NYC/Boston/DC on east coast because they wanted to be in NYC/Boston/DC. Many of my friends went to said schools for psychology/film/etc type programs at schools that as an engineering major were decidedly for me "safety schools". Many of these were 50% admit then and 10% admit now. I don't think they suddenly became as difficult as MIT.

Meanwhile many engineering schools (the types with Institute in the name) that were in cold rural boring areas had higher acceptance rates than my "safety schools" in Boston.

This trend has seemingly only accelerated. For example picking 3 schools in Boston to isolate just the difficulty feature:

* In 2005 MIT acceptance was ~16% vs 4% now (1/4).

* BC was ~30% vs 15% now (1/2).

* BU was ~68% vs 11% now (1/6).

So theres an argument that the harder the school the less dramatically its acceptance rate has compressed, while desirable good but not super hard schools (which have fluffy majors available) have had a larger effect.