Kun is currently not canon.
The Art of Solo mentioned that Dryden's desk was made from stone that he had "plundered from the Sith temple of Exar Kun":
https://i.imgur.com/JHlODeO.jpg
But when Matt Martin was asked about it, he just referred back to what Solo: The Official Guide had to say:
https://i.imgur.com/VDIbxFl.jpg
https://i.imgur.com/D2Kfa5v.jpg
"Dryden's desk contains a slab of obsidian stone that he plundered from a Sith temple." -- No mention of Kun. Martin also went on to say that we shouldn't rely on story material from the Art books too much.
Leland Chee was also asked the same question:
https://i.imgur.com/96LK5a5.jpg
Firm "no."
That being said, now that many other ancient Sith have been canonized by the RoS VD and whatnot, I'm sure Kun will indeed pop-up in canon at some point.
