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in K&R string constants could be modified
Every compiler has a writable strings flag for backwards compatibility.
Everything else you cited is backwards compatible. You are confusing forward compatibility with backwards compatibility.
Was Perl 5 backward compatible with Perl 4? Or C11 with C99? Or even C99 with C89, or K&R C?
Yes.
It's not a matter of lead time in announcing it. If it's not backwards compatible then it's a different language. I can still run 20 year old C code in a modern C compiler.
I've got nothing against Python3. But recognize that Python3 got itself into this mess by being incompatible with Python2. It's different language, albeit superficially similar to Python2. Python3 should have been given another name like "Bob" or something. Ditto for Perl6 vs Perl5 and Angular2 vs Angular1.