There is a significant difference between "we've never tried it on XX and so we don't support it", and "it used to support XX and now doesn't".
You are trying to cite cases of the former but it is actually the latter. Without further explicit clarification, that invariably means "we have decided to start doing things that do not work in XX", ie. that people can definitely no longer use XX. Those cases are the almost the same. It means "we *may or will* start doing things that do not work in XX". As Boost consists of multiple, almost independent libraries (different maintainers), it means that
those libs will be C++03 incompatible at different times. But generally for Boost devs it means: No more C++03 tests required. That reduces headaches etc and old (pre C++11) code can be replaced by modern one (auto, range-based for, constexpr...) So you can say: "We no longer test C++03 and at some point it will be impossible to be used"