[KronisLV]

> I've encountered a code written in the 12factor style of using environment variables for configuration, and in that particular case there was no validation nor documentation of the configuration options. Is this typical?

This just feels like bad development and isn't unlike being given a random .properties/.ini file with no explanations of what the values mean. Sounds like someone didn't do their job, or the processes to encourage (require) them to do so weren't in place.

> For onboarding new members, I would have thought it preferable to have a JSON configuration, where both documentation and validation of configuration options are provided by a JSON Schema file.

You know, this can work, but then you need your applications to be able to read that file and feeding it in through your container management solution (which many use for a variety of reasons) wouldn't be as easy. Even without containers, you'd still need to look out so you don't end up with 20 JSON files all of which might need to be changed for a new environment.

Honestly, something like JSON5 https://json5.org/ was pretty cool because of added comments, but otherwise JSON is a bit cumbersome to use. That said, some web servers like Caddy have gone for accepting JSON configuration as well, which lends itself nicely to automation, so it's still a valid approach: https://caddyserver.com/docs/json/