I think that we should support the -f[no-]exception and -f[no-]rtti cases for multilib selection. The question is what other flags should be supported as well and how to handle them in maintainable way. For example, I think we should also support -fsanitize since I’m aware of more than one toolchain that ships multilib variants for sanitizer instrumented runtime libraries. I also expect the list of flags to grow over time.
An idea I had is to add a new Clang options flag which would be used to specify which flags should be considered for multilib selection, but I haven’t yet had time to prototype this approach to see whether it’s feasible or not.