This is the main GitHub issue for discussing this feature: Camera boundry · Issue #4802 · CesiumGS/cesium · GitHub
Looks like the work was just never completed, based on the comments here: Limit Camera's Position by hanbollar · Pull Request #6381 · CesiumGS/cesium · GitHub. If you do get a chance to test this and finish it, I think we’d happy to look at a new pull request!
I think this is ignored when going underground, there is a somewhat related “underground pick distance” (this would control whether you can orbit the camera around points underground) that isn’t yet part of the public API, you can see this discussion here: Camera behavior after update to version 1.70 - #3 by sean_lilley