No, it is not new. The policy is “Trade is the suggested use for the channel, but it does not have to be about trade. Players can discuss other things.” So Blizzard does not enforce “Trade” as ONLY being about Trade.
It is, as you said, a global channel for everyone in cities to talk in. Blizz knows that and is fine with that.
Agreed though that it would have been nicer if they just created a World Chat channel and were done with it.
From one of the many topics on it. Blue post.
That was 2020 so obviously did not have the new Boosting restrictions on it. It should answer the question though about the policy.
Vrak also has another post in that thread quoting back to the older versions of the forums discussing the same policy.
Of course intentionally baiting, trolling, and trying to cause drama in those channels can get someone reported and actioned. Politics and religion tend to be hot topics best avoided.