It's not in the letter but according to another thread on here (can't find it at the moment) the Phuket office were saying that insurance will not be required for extension of stay if the related O-A visa was issued before 2017.

1: Was that / Is that still true?

If  true for Phuket

2: Does it apply to all O-A visas or just those lucky enough to live in Phuket?

If true for every immigration office 

3: Is there an official reference that I can show for an extension in Samut Prakan?

51 minutes ago, turgid said:

If true for every immigration office 

3: Is there an official reference that I can show for an extension in Samut Prakan?

It is true for Phuket however I doubt Samut Prakan will be interested in what Phuket immigration office does. 

It is here on the Phuket immigration volunteers website. https://piv-phuket.com/long-stay-extensions/retirement/

No other office has that rule.

In July, I asked Immigration what health insurance coverage I needed to show for my next year's renewal based on a 2013 OA visa. The date for my renewal was 26.9.2022.

 

The immigration officials who then visited me at home told me that the old rule still applies to me (40,000/400,000 baht). From 1 October 2022, a new rule will apply.

 

The immigration officials handed me a document dated 27.12.2021 called "Order of Royal Police no. 654/2564 - Subject: Amendment to Criteria and Conditions of an Alien's Application for a Temporary Stay in the Kingdom".

 

Point 6 concerns the new regulation regarding health insurance (see picture).

Any idea what "non-Thai social welfare" refers to in para 6.2, above?

I visited Samut Prakan to do my 90 day reporting today (very fast and efficient). I asked about the insurance requirement  for my O-A extension which expires next month. I said I understood  from the internet it had changed from October 1st 2022. Information gave me a printed set of requirements from 2020 which included no insurance at all. When i pushed a bit they then got someone on the phone who said I needed to speak to an immigration officer which was arranged. 

After consulting with several people the immigration officer said the requirement is the same as last year - 40/400 and they have received no instruction to change it.    Pretty sure it will filter through eventually but presumably if you went in today they would process you based on 40/400.