Obligatory Sold a Story podcast link.
I can't help but feel that a lot of this is deliberate, the end result of decades of dismantling the public education system to further divide kids into the upper class in private schools, religious fundamentalists in home schooling, and everyone else abandoned to keep the population uneducated and in worse economic precarity.
Somebody please tell me that the kids are alright 
Here is a more official study (that had a less dire analysis) : https://nces.ed.gov/pubs2019/2019179/index.asp
However even this shows only 12.9% of American adults are at a level 4/5 literacy. In the footnotes it stated these levels were combined because only 2% reached level 5 across all surveyed countries.
I'm not sure how America defines those levels, but this is how the UK does:
Level 1: Adults can read relatively short digital or print texts to locate a single piece of information that is identical to or synonymous with the information given in the question. Knowledge and skill in recognising basic vocabulary, determining the meaning of sentences, and reading short paragraphs of text is expected.
Level 2: Adults can make matches between the text, either digital or printed, and information. Adults can paraphrase or make low-level inferences.
Level 3: Adults are required to read and navigate dense, lengthy or complex texts.
Level 4: Adults can integrate, interpret or synthesise information from complex or lengthy texts. Adults can identify and understand one or more specific, non-central idea(s) in the text in order to interpret or evaluate subtle evidence-claim or persuasive discourse relationships.
Level 5: Adults can search for, and integrate, information across multiple, dense texts; construct syntheses of similar and contrasting ideas or points of view; or evaluate evidence based arguments. Adults understand subtle, rhetorical cues and can make high-level inferences or use specialised background knowledge.
Is that abnormally low on a historical/geographic scale?
Lower than average (overall, not just 4/5) in the linked study in the footnotes.
https://read.oecd-ilibrary.org/education/oecd-skills-outlook-2013/the-supply-of-key-information-processing-skills_9789264204256-6-en#page9
It seems like several of the articles covering this are being a bit doomer about all of this now that I've seen the actual study. It's funny how the articles complaining about literacy also refuse to cite things properly.
I think there was one done by the dept of education in the US too which had worse results, but I'm on my phone and dont have time to find it.
What I'm usually more concerned about is how many adults I've met have terrible media literacy, and get most of their information from TV news, Facebook videos, and YouTube videos and take all of that in uncritically. But that's a different problem. I know very few people who read anything more complicated than a cheap novel though, which isn't great, but that's all anecdotal.
Nobody wins the Nobel for proving the Null hypothesis (which is a shame, in its own way).
That's what is within easy reach.
I'll recommend this or that source to my elderly mom, for instance. But she struggles with the medium of PDFs and is totally beyond podcasts.
Meanwhile, MSNBC is just... right there on the TV. YouTube is consistently in the front page of a Google search. Facebook shoves headlines right into your feed full of grandkid pictures.
Complaining about media literacy is fascille when the "bad" sources are prominently on display while the "good" sources are buried in the weeds. At this point, it isn't a media problem but a technology problem.
Yeah, that's a good way to put it