Most if not all systems had threads before POSIX Threads. POSIX Threads brought a standard interface to replace the chaos that reigned. It took many years for t

Most if not all systems had threads before POSIX Threads. POSIX Threads brought a standard interface to replace the chaos that reigned. It took many years for the ridiculous amount of thread API portability cruft in Unix ecosystem software to be excised. You saw something similar with atomics until C11 and C++11 atomics displaced old interfaces, though that switchover seemed to happen much quicker, perhaps because it was principally a userspace issue and you could bring newer toolchains and binaries to older platforms.