What Is the Deadline for the Automatic Emergency Tourist Visa Extension in Thailand?

Never heard that such a thing started.

But if you mean that people with an Admitted until or permitted up to date stamped in their passport after 26 March 2020, can stay inside Thailand until the announcement how this period ends and how they all have to get a new visa or valid stay. Than it ends as instructed in that new announcemet

Just so you're clear there was NO automatic emergency tourist visa extension program.. It was an amnesty that waives overstay fines for people trapped here until no longer than the 31st of July.

AND

Before you even ask, no one knows what will happen when the amnesty runs out. Watch for the announcement from the government about what amnesty holders need to do (which will probably be announced closer to when it's running out ;) )

The Amnesty program runs until the 31st of July