They just renamed "system" to "developer" for some reason. Their API doesn't care which one you use, it'll translate to the right one. From the page you linked:
> "developer": from the application developer (possibly OpenAI), formerly "system"
(That said, I guess what you said about "platform" being above "system"/"developer" still holds.)
You can use the "developer" role which is above the "user" role but below "platform" in the hierarchy.
https://cdn.openai.com/spec/model-spec-2024-05-08.html#follo...