Many articles about the US national emergency alert test on Wednesday state that it's not possible to opt-out on mobile phones. However, this is apparently not true, there is an option in GrapheneOS to turn it off.
Discussion
What kind of emergency could affect every citizen of a nation as large and geographically diverse as the United States?
History has examples, such as FDR's annoucements in March of 1933 when all US banks closed for the proclaimed Bank Holiday.
Confirmed that this option works, as of Wednesday, October 4th 2023.
I have not confirmed, is this option available on phones that have been bought from a carrier with the stock OS, or is it disabled?