Also, every website would have to have a way to do this. That's a massive bootstrapping effort that, you can't change every website in the world very easily, there'd have to be a very massive advantage for them. I don't see the evolutionary pathway to get your suggestion implemented.
Besides, this already exists anyway, it's basically:
This has been a very gradual change. The earliest announcement I can find is from 2018[1] but I'm pretty sure it was in the works long before. That's more than five years to implement a technology that browsers and servers at the time already had known how to do for a decade or more.
Besides, this already exists anyway, it's basically:
brong+secretkey@fastmail.com
Or if you have fastmail style subaddressing:
secretkey@brong.fastmail.com