Yeah, I've wondered about precisely this too!

I've been following, as best I can, the research into writing skill amongst students from K to 12, and the single biggest problem seems to be one that is, alas, the most intractable: Schools don't devote much attention to teaching the craft of writing itself -- and in fact teachers themselves aren't always trained in much in how to teach it (and are juggling with classes too large to give close attention to individual students). Steve Graham, who's a longstanding research in literacy and how kids acquire it, did a really good meta-survey of what we know about why and how kids learn to write well (or fail to write well), and that was his conclusion -- https://journals.sagepub.com/doi/full/10.3102/0091732X18821125

It seems clear to me that tech has got to have some effect on how kids do or don't learn to write well formally -- things like spellcheck and grammarcheck included!

But I suspect Graham's survey of the research here is correct; the impact of how schools do or don't teach this stuff is overwhelmingly larger. As is income: The other thing the research finds is a pretty linear relationship between income and writing facility ... kids from wealthier families have more resources (and probably live in districts with better-funded schools) and routinely score much higher on tests of writing quality.