Mod Neena recently clarified the rules about advertising Twitch channels in-game and on the forums:
Original message details are unavailable.
We have no issues with players posting their Twitch Channels/Usernames for other players to follow you or your channel. All we ask that you
don't post the full link
.
So, for example, you can say:
Follow me
@JagexNeena
But not:
https://twitter.com/jagexneena
Same with Twitch. Does that make sense?
If not, let me know - but yeah, we have no problems with you communicating your Twitch channels and Twitter tags in-game.
The reason you can't post a full link is because we don't want to encourage users posting or clicking any inappropriate links.