At least in Sweden those stickers only stop the unaddressed leaflets or catalogs they put in the mailbox. If someone printed your name and address on it, the junk will still be delivered.
In the Netherlands too unfortunately, and this is being abused on a small scale by companies with bigger pockets to just purchase an advertising package from the privatised Dutch post (PostNL they are called nowadays), and have them deliver spam with your address and 'to the residents' on it. It's a spam loophole politicians so far have refused to close, and which PostNL defends as being perfectly legal and part of their business offerings. I think I get one of those each month — it's much more expensive to send than normal flyers.
I've started an experiment with these: I just repost them in the postbox and see how many times they come back to me. You can always put them in the postbox with a 'return to sender' on it, but if you leave it as is it seems to come back to you like a paper boomerang. I've had one round-trip now for UberEats; I wonder at which point it'll get taken out of circulation?
Market liberalization in the nineties. It happened all over the globe in various degrees; it was the trendy thing to do. Not all of it was bad, but a lot of harm was done as well.
With postage, letters still get delivered reliably, and postal workers seem fairly paid, but the parcel market is a cluster fuck of a small number of international parcel companies (some home grown from our former national postal service) that have done exactly what economic laws permitted them to do. They are somewhat regulated and high profile, so they came up with a system where subcontractors hire the actual delivery guys and own the vans and stuff (all in the parcel company's livery of course), and the parcel company pays them for each package delivered. Gradually the pay got worse due to competition, and those subcontractors that can survive either get paid too little under increasing pressure (due to covid leading to more packages ordered), or, as happened in Belgium this year, they hire illegal aliens and underpay them.