I think what you are talking of is number of test cases failed or spec coverage affected with a particular refactoring effort.
Eg: If you break the addition operator, you are likely to break all while loops in a program, that is likely to break nearly everything. Therefore a stability policy for any such non backwards compatible release doesn't make much sense.
Imagine that Perl 6 and Rakudo have reached the point that it's very unlikely that such a basic problem would occur but might. More generally, issues are much more likely to arise than with a mature product such as Perl 5.
If you were writing Perl 6 code that runs under Rakudo, wouldn't you want to know what the Rakudo team thinks about such things? Wouldn't you want to know whether they'd consciously break the addition operator and how much notice you might get if they did, and what happens if it occurs accidentally, and so on? Wouldn't you want to know what explicit commitments they are making in this regard? Wouldn't you want to participate in discussions about this?