I've understood that, in order to make IIS read a new environmental variable, I have to reset it via an administrator's command:
iireset /restart
I know that one can also restart IIS from the admin panel at server level. More context to my complete web app setup in my previous question/answer: in brief it is a Python Flask app running on IIS through WSGI and FastCGI.
What is still surprising me is that now, if I delete a previously created environmental variable, IIS keep seeing it, even after restarting it, as done above.
Why is that possibly happening? And how should I delete the variable for IIS?