For what it may be worth :-
I returned to Thailand on a Multiple Entry O-A visa on 31 OCT 2019 - with a Re-entry Permit, because Immigration had told me that this was needed if leaving the country for more than two weeks, as was the case.
Shortly after reaching my home destination, I went to Immigration on 11 NOV 2019, both to report my return and to apply for a 12-month O-A Visa Extension.
Immigration asked me if I had health insurance.
Answer: "Yes."
"Is it with one of the 8 insurance companies approved by the Thai Government?"
I knew nothing about that: but was told it was a new regulation introduced in April that year.
I said "OK, give me the details and I will insure."
Reply: "It is too late. The insurers will wish to check on your health and the policy will not be issued by the Visa's expiry date." (25 NOV, 14 days later)
I said:"What can I do?"
Answer: "We can give you an O Visa in place of the O-A Visa. You do not need health insurance for that."
So an O-Visa, without the government-approved health insurance, is what I now have.
Incidentally, when out of curiosity I studied the approved insurance policies, only two out of the eight companies would insure a person over 70 years of age, and the remaining two would not insure beyond 75, unless you were already insured with them prior to that cut-off age. And the annual premium was about 130K baht for the minimum cover of 400K + 40K baht.
Coincidentally, for approximately the past seven years I have had, on term-deposit here, 1,250,000 baht plus a bit of accrued interest. 800K for the obvious reason and the remainder in case of dire necessity.
Of course, rules may have changed in the past couple of years.