In 1995 I volunteered with a literacy agency in Memphis TN and learned then that the adult illiteracy rate in the US was around this same rate. It's been awhile so I don't remember the exact number but this is close enough that I think it's fair to say it hasn't changed in nearly 30 years. Same for the number on sixth grade reading levels.
It says a lot about our cultural and political priorities that we've not been able to improve these numbers. https://mstdn.social/@DemocracyMattersALot/113467910852523422