By AI watermark, I mean intentional mechanism that allows to distinguish the ChatGPT texts from other human texts. (E.g. preferring atypical words of some sort). If someone find a statistical method to tell the difference, it would not count, the mechanism needs to be intentional.

Also, if it is added later, it would not count, it needs to be present in ChatGPT around 1/1/2024 already. However, if there is public information that the watermark is there and it remains unknown since when, it will be assumed it was there from the beginning. (I will wait cca 1 month since the news for the information about when it was implemented. If there is no such information, the question resolves YES.)