Advertising of any third party sites is against the Blizzard rules. If reported it would be removed and the account may be penalized. It is not ok to advertise a twitch stream, youtube, website, or even girl scout cookies on Blizzard’s platforms.
If Blizzard wants to promote a streamer, they will do it themselves via their own media.
If you see any website advertising in LFG, chat, or the forums feel free to flag it.
And here is a Blue post from a quick google that backs the - no advertising- stance.
Blizz has never allowed advertising of websites/streams on their platforms. The change for advertising was for just the actual in-game run. A person can offer their, or their guild’s services for gold but it has to be in the Trade Services channel only.