It is not the linux standard C++ ABI, it's just the defacto standard ABI because of gcc's former dominance and clang intimidating the ABI. And I broke things in the past, where I had to recompile stuff, due to different compilers (clang, clang+libc++, gcc in different -std=c++ modes) producing not 100% compatible outputs.
You can say it's good enough (most of the time), but it isn't really a standard, unless I am mistaken.
The Itanium ABI it is not just whatever GCC does; while it is not an ISO standard, it is an intervendor ABI documented independently of any compiler implementation and changes are agreed among compiler teams. It is continually updated to track the C++ evolution.
The standard library ABI it is not covered the the Itanum ABI (outside of some basic functionality), but it is defined necessarily by the platform. For linux that would be libstdc++.
The LSB references the Itanium ABI and defines libstdc++ as the ABI for the C++ standard library on linux platforms; it is again not an ISO standard, but it is as close as you can get on Linux.
And of course the C++ ABI being a very complex and both the ABI document itself and compilers have bugs from time to time, especially if you live close to the bleeding edge.
although it might the facto be, at least for AMD64, I wouldn't say it is the official standard ABI of all unix systems. But it is the standard ABI of Linux based systems, at least those that claim to conform to the LSB.
You can say it's good enough (most of the time), but it isn't really a standard, unless I am mistaken.