Perhaps, the template used are fairly generic, so it usually won’t be that specific. Sorry for the confusion.
The policies involved tend to be a bit vague, as they are intended to cover a wide array of sins. The in-game Code of Conduct often focuses on inappropriate behavior such as using profanity or inappropriate subject matter, toxicity. As the others pointed out, it does specifically list “advertisement” it in the Social Contract, which covers both suggested behaviors and outlines social policy.
Outside of that, you are most likely looking for a section of the EULA. My best non-legal guess would say Section 1, subsection C part iii.
communicating or facilitating (by text, live audio communications, or otherwise) any commercial advertisement, solicitation or offer through or within the Platform;
Much of it comes down to impact on your fellow players and what makes sense. Does it make sense that we would allow you to build up your streaming profile within the game? Is that good for the in-game community? Or do you think that would just lead to budding influencers spamming public chat with their YouTube, Twitch and other social media platforms?
I hope that helps.