What are the current requirements for maintaining a retirement visa in Thailand regarding bank balance?

800.000 2 months before plus 3 months after. The rest if the year at least 400.000

You have an Extension of Stay based on being over 50 years of age. The new requirement is: at least 2 months before the application date till at least 3 months after application date and the rest of the year not less than 400.000 Thb. If you apply in Jomtien, you get a copy of these new rules and you sign that you understand and must come back after 90 days to show the bank balance is still 800.000 Thb. See example:

For an extension (>50 using banked money) it is 800k for 2 months prior and 3 months after, and not less than 400k for the remainder of the year.