Incubating --- what does it really mean?

I’m working on migrating from 5.x to 6.1.x, and finding a lot of classes having this annotation:

Incubating - which is described as:

 * Marks certain of packages, types, etc. as incubating, potentially
 * recursively.  Incubating indicates something that is still being
 * actively developed and therefore may change at a later time; a
 * "tech preview".
 * <p/>
 * Users of these types and methods are considered early adopters who
 * help shape the final definition of these types/methods, along with
 * the needs of consumers.
 * @implNote Defined with RUNTIME retention so tooling can see it
 * @author Steve Ebersole

So - with the amount of things that have this annotation, how likely is it that subsequent 6.x versions will actually have non-backward compatible changes with prior 6.x versions? Asking because I’d like to know the potential risks involved.

We can’t say how likely it is as we simply don’t know. We won’t just change contracts for fun though. We usually will only change the contracts if there is a real need. Even then, we often try to do changes in a backwards compatible way, but we don’t want to commit to full backwards compatibility.
Having said that, I think that consuming these contracts should be fine, but if you plan on implementing them, you might be more affected by changes.