The Thai government proposes implementing a new health insurance rule for all long-term non-immigrant residents eg. retirees. They must (once it becomes an Immigration regulation) show proof of COVID-specific health insurance to a value of USD100,000. As it's generally impossible currently for anyone over 70 to take up a new health insurance policy in Thailand, the government has acknowledged that policies purchased from overseas insurers will be accepted.

There's a similar requirement for all tourists already in place; those over 75 equally have limited options for buying such cover although tipinsure.com seems to provide such cover (roughly 50 baht per day)

https://www.bangkokpost.com/thailand...for-long-stays