People arguing against changing struct layout rules are probably mainly interested, then, in maintaining backward compatibility with older Rust, itself.
Anyway, same policy applies: annotate your structs "packed" anywhere performance matters and bitwise compatibility with other stuff doesn't.
Rust does not keep backward compatibility like that. In the absence of any statement forcing a specific ABI, the only guaranteed compatibility is with code that's part of the same build, or else part of the current Rust runtime and being linked into said build.
Even in Rust, mapping memory between processes, or via files between past and future processes, happens. Although structure-blasting is frowned upon in some circles, it is very fast where allowed.
But... when you are structure-blasting, you are probably also already paying generous attention to layout issues.
Anyway, same policy applies: annotate your structs "packed" anywhere performance matters and bitwise compatibility with other stuff doesn't.