There are other evidences of persistent barriers to skills and rationality attainment though, with fairly strong evidence.

US high school graduation rates reached a pretty firm plateau in about 1950, having risen from six percent in 1900. Much of the ballyhoo over secondary education (test scores vs. graduation rates) has involved trading one against the other, though improvements in fundamental living standards for the poorest (well mother/baby care, nutrition, housing, environmental contaminants most especially heavy metals, reduced general precarity) have also contributed greatly, as has equality of access. All of these being before pedagogical factors are considered --- raiseing the floor is the most cost-effective way of raising numerous population averages.

Hiher-education attainment similarly shows some resistance to expansion, as well as questions regarding comparability over time. Bachellors, Masters, and PhD inflation seem likely. There are also cases where standards seem to have tightened somewhat: there was a biography of a 19th century American who was admitted to the bar in a Southern state on the basis of a brief interview, but who declined the practived on account of the obviously lax standards evidenced. (Ran across recently via Wikipedia, though the details escape me.)

Back to literacy, the US seems to struggle to achieve ~95% at a minimum, the number I'd initially written above, though by its own measure (assuming all those unable to participate in the assessment are illiterate) as low as 91%.

The UNESCO values strike me as somewhat suspiciously high. I'm not sure that's warranted suspicion, but it suggests investigating methods more deeply.

Complexity expands to meet all constraint boundaries.