Environment variables can be useful. The first version of some code can be oblivious to environment variables and the second version can use them for feature flags. Because it is entirely optional to provide these, the second version of the code can be deployed to the first version’s infrastructure, and the first version will happily ignore environment variables set in the second version’s infrastructure.

As a tool for incrementally moving heterogeneous parts of a system forward without needing atomic leaps, the use of environment variables as optional flags is very useful.

I just wouldn’t even start from there. Instead I configure everything in code, in one repo, with all new deployments appearing as new clusters where the only thing they have in common is that they take production traffic. This kind of production infra is very useful but there are downsides for sure. It makes database schema changes, for example, something between very hard and anathema to execute.