So I ignored this manga for the longest time because I saw "mute" and I didn't really want to read any disabilty/drama/angst. I've already watched and read Koe no Katachi, and while I thought it was quite good, I didn't exactly want to do it all over again. I was finally swayed when someone posted some pages of it on a Discord server I'm in so I became interested.
Now I'm way late to this conversation but I just wanted to throw in my 2 cents regarding using your phone to text/LINE/etc. While this was for absent for most of Koe no Katachi, for years now many series like Yuru Camp does this extremely frequently even though none of the characters are disabled, to the point where it makes me think "damn kids these days always glued to their phones and not talking face-to-face enough!" (even though I do exactly that myself, even before the pandemic, so I'm not THAT old)
Others have raised the points that "the notebook is cuter" and "being able to communicate through their phones would minimize the use/impact/drama of the mind-reading powers/muteness", which I think are all plausible. I'd also suggest maybe the author didn't grow up on texting/LINE/emails and just isn't hip with the times, so to speak. Most writers can only write what they know.