Most of ORE is written in C++11 (but not C++14 or 17, so it’s modern, but not cutting edge), as it says in the FAQ this is for convenience, features like autoboxing, foreach loops, etc. There are a few lambdas used, but nothing that would be difficult to back port to C++98/03 (e.g. no use of std::thread or anything like that)
That said, there are no plans to support earlier versions of C++, if you are a windows user, you need MSVC 2013 or higher, this was released over 3 years ago. For GCC it is version 4.8.1 which again is over 3 years ago, so it should be accessible. Restricting ourselves to C++98/03 would be a retrograde step and it’s hard to maintain for windows users (MSVC does not have an equivalent of gcc’s –std= flag).