Completely unrelated to AC, I was just googling up what Steven Speilberg's son was up to as Tim Schafer was mentioning that he's in the industry now in his "Tim plays Day of the Tentacle" video that I was watching. And apparently he's a game designer on AC Unity. And he did this interview which talks specifically about the new free-run mechanics:
https://www.youtube.com/watch?v=cMxZzsLnSWk
So basically it sounds like
RT/R1 = run
RT+A/R1+X = controlled ascent
RT+B/R1+O = controlled descent
I do wonder how climbing up buildings will work-- if just holding the run+ascent buttons will let you climb up a building or if you'll need to hit the jump button every time you want to jump a big distance up a building.
My big complaint of AC3/AC4's core mechanics was that the simplified free-run basically made climbing buildings not as satisfying and made any platforming sequences unnecessary in the games. Hoping these new mechanics will bring platforming back in full force.