Earlier quoted context omitted.
Not just API compatibility. To maintain ABI compatibility as well, at this point they are working with a fork of a really old gcc version that still implements this particular C++ ABI[1]. [1] https://en.wikipedia.org/wiki/Haiku_(operating_system)#Compa...
That's only on the 32 bit version, and even there they also use GCC 8.
It would explain, why I couldn't find that "how to build the toolchain" page anymore through some quick browsing and posted the Wikipedia reference instead. I guess somebody(TM) should update the Wikipedia page then.