We can also support nested directories using the nested delimiter e.g. a file path like db/password could be mapped to db__password, which would write that into a nested model db with field password.

            # Transform keys for nested delimiter support
            if self.secrets_nested_delimiter and self.secrets_nested_delimiter != os.sep:
                transformed_secrets = {}
                for key, value in secrets.items():
                    new_key = key.replace(os.sep, self.secrets_nested_delimiter)
                    transformed_secrets[new_key] = value
                secrets = transformed_secrets

I see that the docs state that this is already supported, so I'm not sure.

Not sure what you mean here)

In my implementation I added this, but don't really remember why 😅

Maybe it was to also support the nested delimiter within the secret name i.e. a secret file named db__password.

Oh, I see, you mean the case of both nested directory and file name prefix. This is already supported: https://github.com/makukha/pydantic-settings/blob/c39546cbe742855b7c96f2f9b5bcc23e64f7e8f1/tests/test_source_nested_secrets.py#L130