I don't think it's canon, just that there's a lot of subtext. People are just saying it's canon in a shipping. (although at this point they are pretty much married even though there's no canon romance)
It's stated at the official page of Moriarty. In each character's definition, particularly in Sherlock and William, says they are each other's forbidden fruit and so it was confirmed then.
I am pretty sure the author is hardcore fujoshi like the rest of us, I mean the way she write the interactions between the two of them is soo freaking romantic in all aspects; the teasing, the mindgames, their expressions,... it's literally confirmed that they are soulmates, I mean even the other characters are aware of their special bond (their marriage vow as noted by billy ⁄(⁄ ⁄·⁄ω⁄·⁄ ⁄)⁄ )
When and where was it confirmed that sherliam is Canon? Idk if I had missed a page or misunderstood smh bc I was reading chap pretty late and English isn't my first language. But I've been searching for like source of it and I cannot find it I go to the comments underneath the videos and people are not asking questions or saying source or anything they just celebrating ship coming true. And I am also happy, bro i was acting like child on caffeine when I found out, but can you pls give me the source, like anything even brief explanation will do ┗( T﹏T )┛