All other versions state it's not. I asked ChatGPT-5 and it responded that it's it's prompt (I pasted the reply in another comment).

I even obfuscated the prompt taking out any reference to ChatGPT, OpenAI, 4.5, o3 etc and it responded in a new chat to "what is this?" as "That’s part of my system prompt — internal instructions that set my capabilities, tone, and behavior."