@Philip *********
I'm not entirely sure why they were willing to make an exception in my case. They initially informed me that they were not issuing these letters to any Canadian citizens, and they advised me to return to Canada or somehow convert to a long-stay visa for Thailand, such as a retirement visa.
But a short time later, I learned that the American embassy had begun issuing such letters, and I wrote to the Canadian embassy a second time to ask if they were going to follow suit and change their policy.
They informed me that their policy had not changed, and they were not issuing these letters. However, something in my email describing my situation must have caught their eye, because they made an exception for me and issued a letter.
I don't know for sure, but I believe the deciding factor was that I had not maintained a residence in Canada for nearly three decades. I've lived almost my entire adult life in other countries. And with no home to return to in Canada, it would be difficult to comply with the self-quarantine measures. I supplied other reasons and arguments, probably no different than those of other people, so I'm not entirely sure which argument swayed them. I was grateful, however, to receive the letter.
What eventually happened in your case? Since you could not get this letter, I assume you had to leave Thailand or apply for a different type of visa?