They're basically cannon, I've heard.
*spoilers*
They are not explicitly canon, but one can argue about whether or not their relationship has an underlying romantic nature. SHJ and HYJ did part in the main stories' ending because of some special circumstances, but they are still in contact with each other and is actively working together in the side stories (I don't know much about whether or not they've physically reunited though since I haven't caught up to the latest chapters).
There are *LOTS* of their moments, by lots, I mean like a ton of them— almost every single chapter, possibly because of how SHJ alongside the siblings, is a big plot force as well. If the webtoon follows the revised E-Book version, both of them would be sleeping in the same bed the next arc, tho I don't know if they will adapt that into the webtoon.
I suggest reading the novel because there are differences from the webtoon and also just because of the homoerotic moments between those two.
*side story spoilers*
From what I can tell from random twitter blurbs I think they did physically reunite in the side stories and Yoojin said he was okay with kissing Hyunjae on the forehead/cheek?? And something about Yoojin proposing but I was lost at that point
Man I wish I could read the side stories
l
Someone needs to spoil me the ending of the novel involving Hyunjae and Yoojin PLEASE. If not, please spoil me more of their moments together