The United States Department of Education has performed several detailed assessments of adult literacy since the 1980s. Whilst many countries report literacy rates based on very basic reading abilities, the US studies differentiate by skill level, with results that are occasionally pointed at by critics of various aspects of US institutions. However, there is a strong argument to be made that it's not about the US having especially poor literacy attainment, but markedly accurate assessments of that attainment.

A somewhat similar study is the 2016 OECD report (based surveys conducted in 2011--12 and 2014-15, involving 215,942 and at least 5,000 in each country), "Skills Matter: Further Results from the Survey of Adult Skills"

https://www.oecd-ilibrary.org/education/skills-matter_978926...

Jacob Nielsen addressed that in the context of computer UI/UX requirements for general-use application and website design, "The Distribution of Users’ Computer Skills: Worse Than You Think":

Across 33 rich countries, only 5% of the population has high computer-related abilities, and only a third of people can complete medium-complexity tasks.

https://www.nngroup.com/articles/computer-skill-levels/

I've referenced both in my own essay, "The Tyranny of the Minimum Viable User", which acknowledges both that general-use systems must be generally-usable, and that this requirement reduces functionality for advanced users by numerous mechanisms. That conflict seems inevitable.

https://old.reddit.com/r/dredmorbius/comments/69wk8y/the_tyr...

There is additional research in the field of cognitive development with similar trends (see generally Jean Piaget).

For those interested in a detailed, long-term, large-picture view of the US educational system, see US Department of Education, 120 Years of American Education: A Statistical Portrait (1993):

https://www.google.com/books/edition/120_Years_of_American_E... (downloadable PDF)

It's worth noting that in 1900, the high-school graduation rate within the US was 6%, of whom 60% were female. That rose to 76% of 17 year-olds by the 1962-63 school year, and has fluctated around that level since. (Subsequent attainment of a HS diploma or equivalent general education diploma (GED) certificate by age 25 is somewhat higher.)

The upshot from all of this is that literacy and skills of the general population are likely far lower than you would expect, in both technical and nontechnical areas. Whether you're designing UI/UX, writing journalism, running for elective office, or engineering or combatting propaganda, these facts must be taken into consideration. Blind faith that "all the children are above average" will doom plans. Though so too may presumptions that literacy or technical skills are themselves direct measures of intelligence.