While I am sure that thousands of hours have been spent debugging threaded setenv() attempts (and developing & discarding Annex K), it is clearly not a problem

While I am sure that thousands of hours have been spent debugging threaded setenv() attempts (and developing & discarding Annex K), it is clearly not a problem that needs a solution. Languages that compile to C need be careful not to promise thread-safe implementations of POSIX or C functions that are explicitly documented as not reliably thread-safe, including setenv(). The author seems to want to change C, and POSIX, so that Go can reliably do so.