But for that you need a personal domain and the ability to configure something like that, whereas everyone in a corporation that runs their email through Google can use the +abc thing out of the box, and I've seen it used a ton. The goal with measures like that is usually not to catch every single one who tries to game the free tier but to ensure it's more inconvenient to do so than just buy it for most people, and typically filtering +abc emails will be just one of several different measures taken to that end.