Do you have a single example of this? We keep very close tabs on breakage in the wild even, and especially, for changes we were allowed to make (unlike say GCC, which happily breaks code if the language standard says the code never should have compiled in the first place). There are times when we've refused to make changes that we were allowed to make (because they were changes to unspecified behavior, which C/C++ has more of than Rust, and which GCC/Clang changes all the time) out of concern for breaking existing code.
As far as I'm concerned, honestly, this is just FUD.