Many old-school unixes were developed in the single-thread days. When they added support for threads they were an opt-in feature, like the above macro definitio

Many old-school unixes were developed in the single-thread days. When they added support for threads they were an opt-in feature, like the above macro definitions, and using libraries like libc_r.so instead of libc.so, and so on. AIX even had compiler wrappers like xlc_r which set the relevant preprocessor macros and linked in the correct runtime libraries. When the Linux glibc 2.x/NPTL ABI was taken into use, it was clear that threads were not a passing fad and stuff was made multi-thread (to the extent the API can enable that, of course) by default, so there was no need for carrying around both a single-thread ABI and a multi-thread ABI.