There were breaking changes between C and C++ (and some divergent evolution since the initial split) as well as breaking changes between different releases of C++ itself. I am not saying these never happened, but the powers that be controlling the standard have worked hard to minimize these for better or worse.

If I took one of my earliest ANSI C programs from the 80s and ran it through a C++23 compiler, I would probably need to remove a bunch of register statements and maybe check if an assumption of 16-bit int is going to land me in some trouble, but otherwise, I think it would build as long as it's not linking in any 3rd party libraries.