I don’t use theme’s, but my experience outside of theme’s is that there have been basically no breaking changes in the 4-5 years (?) I have been using it. I can think of one change that had to do with markdown that effected a handful of people. Seems like there was something else but I can’t think of it.
I also don’t use any plugins - I think once you start using plugins and themes your likelihood of a breaking change increases quite a bit.
All that said I believe the next major release is going to be 4.0 and I would expect that to have breaking changes, I remember reading something about it but don’t remember what it was. I don’t think there is any time-frame for the release of 4.0 yet.