I think that's missing the point. A language can be a moving target while still being backwards-compatible. The textbook example is C++: A C++17 compiler can compile C++03 code, C++11 code and C++14 code [1], so the language is backwards-compatible. Meanwhile, each new release seems to add so many new features to the language and stdlib that idiomatic C++17 code can be wildly different from idiomatic C++11 code and even more so from idiomatic C++03 code.
[1] Except, probably, for corner cases that I'm not aware of because I don't do C++ anymore.
It's not quite like that in Rust though - in C++ modern features replace older features but in Rust that only happens in the detail. There isn't often an equivalent to unique_ptr/shared_ptr changing how you should to all pointer work.
[1] Except, probably, for corner cases that I'm not aware of because I don't do C++ anymore.