alt Hacker Newsyay is a package manager that has been made for AUR. yay is not the official package manager for Arch Linux, pacman is, and it does not support AUR. yay is not installed on Arch Linux by default, its official package manager, pacman, is. AUR is for unofficial 3rd party packages, i.e. "use at your own risk". It has always been the case.
Yes, it is "use at your own risk" but most arch users just install from it without giving it a second thought, because availability of packages in the AUR is the one thing Arch is good at.