It's always been a thing, it's just More prelevant these days than before.Also, this isn't a new thing. My buddy is pretty much illiterate but graduated high school. He's not alone in my age group.
I bet you that 90% of the illiterate adults could learn to read in 6-12 months if they tried / wanted to.
A lot of kids are behind because not everyone learns at the same pace, my kid was reading faster than any in his class, but was behind in math. He didn't understand 1 section, so he was lost for the rest of the year. If they just kept bumping him grade after grade.... He'd still be behind in math and he'd be in grade 12 not knowing how to do most of the work. If he failed grade 1 math and had to goto summer school or back to grade 1 next year... It'd suck for sure, but at least he wouldn't be left behind to struggle in higher classes. I'm lucky enough to be able to have paid the $400 a month for tutoring to catch him up for a year... But a lot of people can't... So their kids just get stuck and lost in the system.
Not everyone who is behind has a learning disability... They just need a bit of extra attention / help to get past a hurdle. I'd rather them lose a grade / year of their life catching up, than go through the next 10 years of school constantly behind because they don't have the foundation to pass the class. Math especially... It's so linear, if you can't pass grade 3 math you can't pass grade 4-12 math. I don't think kids are getting any favors with a free pass.... But I guess we'll see in 20 years when that generation is out in the work force.