They still advertise it on their own website https://on.google.com/hub/?main=retail , so they expect it to be still selling.

> I don't see how Google can control the behavior of retailers of 3rd-party hardware.

That hardware bears trademarks that Google controls, so it's not purely a third-party product. Google may not be able to prevent retailers from selling off their remaining stock, but they should at least be able to prevent the manufacturers from sending any more to retailers, and inform retailers that the products are discontinued.

How come smaller companies like Zyxel and Asus have solved this problem decades ago? Maybe because they support their hardware for years after they stopped producing, so the stock clears?

While it is manufactured by a third party, it's branded as a Google product.

You can't control retail inventory (well, for some products retailers report serial numbers sold, so maybe you can), but you can make relatively solid assumptions that if you shipped the units to the store/retail warehouse, they're either going to sell them or return them within some period of time, maybe 3-6 months. So assume last sale was 6 months after the last shipment.

They can buy the units back from retailers. It's common practice.

They can issue a recall.