It rarely happens because at this point the codebase is so littered with problems that things start requiring long QA, code freezes and once-a-month deployments, and it's impossible to get anything done.
My faviourite configuration pattern for SaaS code: all the configuration for all targets, from local development setup, to unit tests, to CI throwaway deployments, to production is in a single Go package. The current environment is selected by a single environment variable.
Need something else configured beyond your code? Write Go code to emit configs for the current environment, in "gen-config some-tool && some-tool" stanza.