We would need a specific proposal and assurance that nearly all computers can efficiently provide that service. It is more likely in the POSIX standard.
Though it's interesting that threads were added to the standard. Perhaps though they filled a niche that wasn't as well filled as select/poll/epoll/kqueue/etc had already since pthread api is perhaps harder.
I thought it would be best to standardize just a single thread, which should be the basic unit to be embedded in a good parallel-processing model. However, others prevailed.
