As far as I could tell, Signcraft has never been discovered or deciphered before. It's the written language of the Burrowers in Khrysalis, namely the Sugar Gliders and the Mice, and it's used as a secret code to relay information between the rebelling forces.
The player first encounters Signcraft painted on a rock on the outskirts of Tyrian Gorge, a clue left by Sabra SoaringKnife.
Evidently, Sabra doesn't teach the player the details of deciphering Signcraft since 1. The game automatically translates it and 2. That's actually not what the rock says at all. I took it upon myself to decipher the Signcraft that is actually presented to the player, and this is what I uncovered:
The original rock
I turned the markings into a basic substitution cipher to solve it
The results
Given that the painted message ends with a comma, it can possibly be inferred that it is the introduction to Sabra's message. However, there is just not enough space on this small rock to convey the entirety of the character's dialogue.
After solving this initial instance of Signcraft, I found it written in many other places throughout Khrysalis and even in some other random places of the Spiral. Here are some examples:
The entrance to a tunnel found in the Last Wood, Khrysalis
A banner found in Bastion, Khrysalis
Perhaps the most interesting example of Signcraft I stumbled upon can be found in Headmaster Ambrose's own office, which is only accessible during the graduation ceremony quest.
I have found the text very hard to decipher given the poor quality of the image, but I mostly use this image as a means of claiming that Signcraft is a well-known written language, not just a secret code used by Burrower rebels. This tome is also very interesting because it seems to depict a moth with a sword, an NPC that is not actually encountered in Khrysalis.
Using all of the above examples and some other minor details scattered throughout the game, I came up with a translation table that anyone can feel free to use! The alphabet is not fully complete, and I don't think it will be for a while, but I have most of the letters drawn out: