After thinking through and through a potential "environment" feature, I really think it doesn't tie well with windmill. The proper way to do things is to have separate variables and resources. However, there is a much better feature which could solve the same UX pattern and more.
Saved run inputs and history: Next to the run page, the list of all previous runs inputs and some pinned inputs. Those pinned inputs could be shared to anyone or personal, Those pinned inputs could thus be pinned as "Staging" or "Production" when relevant, while being a good fit for many more scenarios.