According to some older comment by jmalak, it is compatible with C++98/C++03 with some C++11 features though, yes, it isn't C++11 compatible (there are some stubs for C++11 that i can see in the code but nothing implemented).
And that's a thing though, SFINAE hinted at it but more recent versions of C++ constexpr and consteval together with relaxed constexpr and "if constexpr" and "if consteval" together with compile-time dynamic typing forced the compiler to more or less include an interpreter in the frontend.
Adding some features of C++11 isn't close to building a C++ compiler capable of 11, 14, let alone 17, 20, 23 or 26.
If you can rip out a compliant parser, it might even be better to start over on a new compiler frontend if your current compiler frontend is targeting at pre-11 level.
