It's a pretty common function in most timeline based media production programs, such as After Effects, but AnimateCC can't do this?
There must be some way to animate that attribute, has anyone managed to do this? How?
with the text tool selected, in the properties panel adust letter tracking:
No, letter spacing is not a natively tweenable attribute. The usual way to do this sort of thing in Flash/Animate is to break text apart into letters, turn each letter into a symbol, then tween each letter individually. I believe there were also plugins for Flash to automate text animations, though I've never used them myself, and I don't know if they'd still work with Animate.
You can also animate this property manually, frame by frame.