To that end, I propose the following:
Boost release 1.82.0 is announced as the last one with C++03 support. If critical problems are found in it post-release, we will issue 1.82.1, 1.82.2 and so on, as appropriate (as C++03 users will not be able to just upgrade to 1.83.)
Boost release 1.83.0 is announced to require C++11 at minimum. This means compilers that have all the C++11 standard headers, and support all the C++11 syntactic constructs and keywords without issuing errors. (E.g. VS2013 doesn't qualify because it doesn't support the `constexpr` or `noexcept` keywords.)
Since people have come up with various creative ways to break all existing code using Boost, and I've been asked off-list "but don't you want to break things", I want to clarify that those are completely outside the scope of my proposal. It is exactly as outlined above, no more and no less. I do not propose "Boost 2.0", replacing the includes with boost2/foo.hpp, replacing the library names with libboost2_foo, bumping the version to 2.0, dropping libraries from Boost, and anything like. As long as a C++11 compiler is used, code that compiles and works with 1.82 should continue to compile and work with 1.83, without changes.