Hallo Dee and Gavin,
As you saw from the FB group, there is no definite answer about this - someone was trying to get hold of the Tourism Association for an answer.
It seems that the "new" rule came into effect some time ago already, in 2017 with an update to the 2003 road law. However, from what we could gather, the 80km per hour sticker is only relevant to "heavy" caravans and trailers, weighing more than 750kg. I.e. you would also need a EB licence (old Code 10) in order to tow that caravan.
It seems that about a month ago, suddenly tourists were being fined for not displaying this 80km/hr sticker - it lasted for about a week, and all around Martin's Drift border post. Since then the comments have died down so no one knows if the "record was set straight" w.r.t. which vehicles should have these stickers, and which not.