That would be in July of 2018. The original post belonged to the old forums, but you can find it here:
https://www.wowhead.com/news=285321/in-game-advertising-policy-changes-for-group-finder
We have a Support article here:
https://us.battle.net/support/en/article/187406
Overall, advertising carries for gold in public channels is fine, as long as they otherwise adhere to our policies.
Basically, the Looking for Group tool was intended to allow folks to find groups to do activities, quests, dungeons, raids, and they had become inundated with ads from guilds and others who were selling carries. Mostly preventing regular users from being able to easily find groups where they didn’t have to pay an entry fee to do the content.