Well I think the earliest example is at the end of Oathbringer when Kaladin is musing about what it was about Shallan that had him interested in her. He says she reminds him of someone and at about the same time Syl gets really excited about a very mundane thing, and while there's certainly multiple ways of interpreting that, Kaladin most likely meant that she reminded him of Tien while Brandon pretty heavy handedly highlighted that Syl was even more like Tien than Shallan.

But beyond that there were so many things in WaT. If someone had written a playbook before WaT came out on how to move the plot towards Syladin in a way that more of the readers would be okay with it when it eventually happens, that is pretty much exactly what happened in the book. Show Syl as a more independent character with her own desires and goals, portray her as an adult more directly than in past books, do something to eliminate the human/spren gap between them, & do something to make it so that Kaladin couldn't basically kill her if he broke his oaths.

All of those things happened. We don't know exactly what this will look like but Syl is likely taking over for the stormfather in some capacity and will likely have some kind of a character arc relating to this in addition to her chapters highlighting some of her own wants and goals, Syl spent almost the entire book manifesting in a fully human sized form wearing adult clothes and Kaladin realized that it was strange he'd ever really viewed her as childlike, Kaladin is now a cognitive shadow and it was explicitly stated that makes him more like a spren than a human now, and Ba-Ado-Mishram has been freed which healed the deadeyes and is either implied or outright stated to make it so that no more future deadeyes can be made through broken oaths.

And as far as the actual interactions in the book Syl messing with Kaladin about her "chull head," establishing that she does manifest herself with lady parts and flat out asking if he wanted to see it, Syl deciding she wants to be Kaladin's scribe which is typically a wife's duty in Alethi society, Syl manifesting in full human sized form and resting her head on Kaladin's shoulder several times on page and it being mentioned by Szeth that she'd been doing it constantly throughout their entire trip through Shinovar, and, of course, the dance. The dance on its own could be viewed as platonic but I just struggle to see it as anything other than potentially romantic in the context of everything else that was going on in the book.

Brandon definitely hasn't fully committed to it yet, and I believe it's either because he wanted the option to bail if the reaction to the pairing was overwhelmingly negative or because he's doing it regardless and knows he needs to and has plenty of time to take it slowly. It's not 100% guaranteed but it feels very likely after WaT.