Since we are targeting C++17 compilers, GCC 6.3, clang 4 and Visual Studio 2017 under version 15.7 will become unsupported after summer
We give this warning to allow people to ready their systems with enough time to update (it's very likely Debian 10 will be shipped on 2nd quarter 2019).
(Visual Studio 2019 release date is on 2nd quarter 2019)
https://clang.llvm.org/cxx_status.html
https://gcc.gnu.org/projects/cxx-status.html#cxx17