I am not sure making things safe by default is a good idea. This always comes with a cost. Thats also the reason why basic data types (array, dictionaries, etc) are generally not thread safe… because its usually not needed or handled on a much higher level.
Its a different story for languages/environments that are supposed to be safe by default and where you have language features that ensure safety (actors, optionals etc) but not for something like libc which has a standard it has to conform to and like 100 years of history.
The problem with `setenv` is that people expect one process to have one set of environment variables, which is shared across multiple languages running in that process. This implies every language must let its environment variables be managed by a central language-independent library -- and on POSIX systems, that's libc.
So if libc refuses to provide thread-safety, that impacts not just C, but all possible languages (except for those that cannot call into C-libraries; as those don't need to bother synchronizing the environment with libc).
A conformant implementation can make a non-reentrant function actually safe under the hood for people that call into it erroneously. Unfortunately, there is no way to do this for getenv/setenv, because of the API they expose (specifically, when environ is accessed directly).
In some cases this is true. In the case of setting and getting env vars, it is not. There is no comceivable reason for making a process that spends any significant portion of its runtime calling setenv() or getenv(). Even if those calls were a thousand times slower than today, it would still be a non-issue.
Its a different story for languages/environments that are supposed to be safe by default and where you have language features that ensure safety (actors, optionals etc) but not for something like libc which has a standard it has to conform to and like 100 years of history.