If the DLO is serious, they could make a sign to that effect and bolt it on the wall themselves. They put up enough other inane notices telling us to do, or not do, things.
Cost basically nothing.
I begin to suspect that, like bike parking on the pier, this is something that the government will "tolerate" but don't want to publicly endorse, giving them the freedom to take away the "privilege" at will.
And I find it absurd that they claim to have instructed the staff to allow posters.
Certainly, when they want them cleaned off, that happens pretty efficiently.
Why would staff bother to this work if they had been told not to?
So I doubt he's actually sent clear instructions to the people concerned.
Anyway, I find it hard to care one way or another. Posters have now migrated to the fence opposite, and many other, less appropriate, places, making a much messier environment. So I will have no problem finding somewhere to advertise a toaster oven.
But the important thing for the government is that the "Welcome Wall" greets visitors with its institutional grey blandness.
Seems an appropriate message from the bureaucrats.
Be seeing you.