Unix's standard error is definitely not the first invention of a sink for errors. According to Doug McIlroy, Unix got standard error in its 6th Edition, release

Unix's standard error is definitely not the first invention of a sink for errors. According to Doug McIlroy, Unix got standard error in its 6th Edition, released in May 1975 (http://www.cs.dartmouth.edu/~doug/reader.pdf http://www.cs.dartmouth.edu/~doug/reader.pdf). 5th Edition was released in June, 1974, so it's reasonable to suppose Unix's standard error was developed during that 11 month interval. By that time, Multics already had a dedicated error stream, called error_output (see https://multicians.org/mtbs/mtb763.html https://multicians.org/mtbs/mtb763.html, dated October 1973). All the same, I'd be willing to believe that Unix's standard error could have been an "independent rediscovery" of one feature made highly desirable by other features (redirection and pipes). It's not clear how much communication there was among distinct OS researcher groups back then, so even if other systems had an analogue, Bell Labs people might not have been aware of it.