Sardonyx

yeah i think they have, which is a shame because I doubt realistically deaf people can read mouths all the time. I liked them communicating with sign language. (the manhwa is literally called sign)

I don't think it's supposed to be as though they've stopped, I think it's just a little taxing to research and draw not to mention it was in large part to emphasize the difference between them and the learning process.

They didn't really stop. You see slight motions of it in the last few chapters recently. But i think it's mostly because the author no longer feels its nessescary for it.
Like they have a full relationship, and can talk pretty openly now. Even Yohan really only did sign language with those he didnt feel comfortable with. And even then he admitted he mostly reads lips.
And that passed when he got to know someone better. Be it coworkers, friends or serious relationships.
We may see it more often, mostly in a work place scenario or in public. But most of their relationship took place in private spaces, so they didnt really see the need to constantly use sign language.
Plus its probably a bitch on the artist to constantly draw that stuff. I dont mean it in a bad way, but 1 wrong gesture and it could have a whole other meaning.
It was probably very time consuming to remember, and draw the correct gestures every time.