To modernise JUCE, we now require the compiler to have some basic C++11/C++14 features (range based for-loops, move semantics, nullptr, etc.). This means that older compilers - specifically VS2003, VS2005 and VS2008 and older versions of Xcode - are no longer supported.
Rest assured that we only require C++11/C++14 language features as opposed to library features. This means that your JUCE apps will still run on - for example OS X 10.6 - you just need a newer compiler.