If you want a cross-compiler ABI, use a C-style subset

Different compilers implement different binary layouts for classes, exception handling, function names, and other implementation details.

Common ABIs are emerging on some platforms freeing you from the more draconian restrictions.

If you use a single compiler, you can use full C++ in interfaces. That might require recompilation after an upgrade to a new compiler version.

(Not enforceable) It is difficult to reliably identify where an interface forms part of an ABI.