Compatibility with carriers also improved a lot a few years ago. Configurations for most carriers are pulled in from the stock Pixel OS. Some US carriers do weird things that depend upon having highly privileged apps bundled into the OS which, for security reasons, GrapheneOS doesnt include. I dont recall AT&T being one of them.
GrapheneOS is very usable and fine as a everyday phone for normal people.
It is not usable for normal people who want to use their phone to stay in contact with others.