What are the rules for demonstrating foreign income for a retirement visa extension in Thailand?

The rules are clear;

For an extension based on being over 50 (retirement) IF you want to use the monthly income method and if you are from a country whose consulate stopped issuing the affidavit of income from abroad notary letters (UK, US, Australia or Denmark) you must show incoming overseas transfers into a thai bank account in your name of at least 65K baht per month each month EVERY month for the preceding 12 months before you apply for your extension.

The transfers must be coded by your thai bank as coming in from abroad (not domestic transfers).

What we're not going to do is debate WHY that's the rule. It's the rule they came up with to allow people whose consulates stopped issuing income affidavits to still use the monthly income method to get extensions of stays.