This is an opinion based answer, my opinion is I would put it down pretty quickly. Anything that is made intentionally difficult to read slows me down, particularly if misspellings don't sound any different than the fully spelled word: "Did u do the thing" sounds absolutely the same as "Did you do the thing", except the first interrupts my flow of reading by trying to see what 'u' is supposed to mean.
If you assume any readers read extremely well and fast, misspellings slow them down.
If you assume any readers read slowly, misspellings won't help them.
Again, this is my opinion, but this is not something that will help your story by making it seem realistic, it will prevent it from being read altogether.
I should point out that people becoming illiterate (or not having books) does not prevent them from having a large vocabulary, and does not make them stupid or dull. In Shakespeare's day (1600), 75% of men and 95% of women in England were illiterate, but they understood his plays.
Grammar and vocabulary are not a result of literacy, but exposure. In modern times we happen to be exposed mostly through print, but there are many routes to exposure, and the truth is that almost all the words we use in daily conversation we learned by ear, NOT in print.
Children learn words and proper grammar years before they learn to read and write. By the time they DO learn to read and write nearly every word they know, they have been speaking for a decade, and many adults NEVER learn to correctly spell nearly every word they know. That does not mean they use those words incorrectly or pronounce them incorrectly.
In fact, learning words from print can impede correct pronunciation; until they hear a math professor pronounce "Euler" many students might think it is pronounced "You-ler", but it is pronounced "Oiler".
So I think your premise is flawed and unrealistic. The loss of all print in the world will not make people start talking like four year olds, their minds would have to be reduced to the level of four year olds. Their language will be just as logically complex and their vocabulary will still be precise: If a doctor means to point out the right iliac vein, that is what he will say. Current doctors have the terminology memorized, and memorization is still a part of the training: You must know every part of anatomy by heart, in all its Latin glory, in order to be a medical doctor. The sudden loss of print would not be a loss of the information at all. The same goes for lawyers, doctors, mathematicians, chemists, historians, grammarians, etc. It is all memorized by someone, and they would quickly transcribe it all to mud tablets marked with pointy sticks if that was needed. If you magically made reading and writing impossible, they would revert to the middle ages practice of rote memorization, before 1500, when most humans in Europe were illiterate, they all had much more memorized than modern humans do today (and the same is true in illiterate primitive times today, some such people learn the names of hundreds of their ancestors and hundreds of stories verbatim, to pass down to their children in oral tradition).
I'm not criticizing your premise as an insult, but to point out it isn't very strong and (IMO) would likely not result in a plausible story. If you want to write some low-brow campy humor, you might get this kind of thing to work in a play or movie similar to Idiocracy.
It might not be a bad premise if the prose is modern and only the characters are of reduced intellect and memory capacity. If you have interactions with any children, a workable premise might be that some disease sweeps the world and leaves all humans incapable of developing any further than the ten year old intellect. Eventually the adults die out and the world is run by adults with the minds of fifth graders.
One turning point that would be interesting is to read what all these normal-thinking adults DO before they die off, to prepare the world to be inhabited by a population entirely composed of ten year old minds.
Another turning point might be centuries later, when civilization has collapsed and all people live and mate in near stone age conditions, and some mutant child is born with a natural immunity to the disease, and develops normally, so after the age of 10, she is, slowly, increasingly smarter than those around her, then everybody around her, then everybody in the world, yet still no smarter than the smartest of us today. But, can a modern adult, alone, control a hundred million adults with the minds of fifth graders all living in tiny kingdoms and separate villages? Maybe, maybe not, it would be interesting to hear her story.