I found some maybe relevant stats on the National Center for Education Statistics website; which will probably be taken offline in the next couple of days.
https://nces.ed.gov/programs/coe/
The National Literacy Institute seems like not a great resource. It’s mostly a consultancy selling programming I think. And their contact info is a Gmail address.
If you have a source you find more credible that shows different numbers, then feel free to share it.
While not a great resource it does match with how public historians are supposed to write. Public history covers statues, monuments, museums and other things. The labels for all of these should he written as if the reader only has a 5th-6th grade reading level and we only expect half of the label to be read. Would not surprise me if it is ever worse than that though.
I really question these numbers. I’m old enough I remember when restaurants had picture menus. In a society where you need to internet on your phone for EVERYTHING, I am deeply skeptical that 1/5 adults is picture menu at McDonalds in 1980-level illiterate.