But that's true of any system that depends on other systems with lax respect for contracts, or with no contract. You can depend on a third-party function get_time() that returns the seconds since the program started, and later on if its maintainers decide to change it to return a UNIX timestamp because they realized the wording was vague enough to allow it, any code that makes the wrong assumption will break.

C++ is, I would say, quite good as far as backwards compatibility goes. The problem IMO is that it's rather complex and some of its features have been misunderstood over time, such as the meaning of volatile or inline, and thus people have been writing subtly broken code that just happened to work when they originally wrote it.