As a really old school UNIX guy I'd agree with this. Programmatic manipulation of the environment is an 'attractive nuisance' in that I feel anything you might be trying to achieve by using the environment as a string scratch pad of things that are different for different threads, can be coded in a much safer way.

I'd be happy to have you copy the immutable read-only environment vector of strings into your space and then treat that as the source of such things.

I think it would be interesting to build all the packages with a stdlib that dumps core on any call to setenv() or unsetenv(). That would give one an idea of the scope of the problem.