US citizens aren't getting letters either. IF this changes, you'll still have to make an appointment online for any business you do at the embassy.

We've heard from several sources that the health certificate policy goes into effect Sunday. I'm not sure if you can get it before you leave, but it's good to have it if you can get it.

No, of course the US Consulate hasn't issued directives to Thai immigration (and I'm not sure why anyone would think that). The US Consulate in Chiang Mai said Thailand's PRIME MINISTER OF TOURISM has issued a directive to Thai immigration that gives them the authority to approve extensions of stay for foreign nationals, including US citizens. Now, does the prime minister of tourism have the authority to do that? I don't know. All I know is this is all we've been told by the consulate. It's not likely you'll get a letter if there are flights out at all. You're also not likely to get a letter in Chiang Mai until after March 31st since you can't even get an appointment. Maybe there will be a cancellation, but I doubt it since there are few appointments even available.