This doesn’t ring a bell. I tried to repro, although I don’t have a 10.9 machine for testing. Instead, I tried building with a 10.9 deployment target and then inspecting the final build output with
nm to see whether the
os_crash_fmt symbol was listed as undefined. With Xcode 12.4 (macOS 11.1 SDK), neither the Debug nor Release mode builds of the DSPModulePluginDemo seem to require this symbol at all.
Perhaps you could try running
nm on all of your plugin object files and dependencies and check whether any of these are depending on this symbol. At least then you’ll know where the symbol is being used, and you may then be able to take steps to avoid using the symbol altogether (adjusting deployment targets of dependencies, removing dependencies etc).