Do I need to show international health insurance when applying for a 1-year extension of a retirement visa in Thailand?

As you're on a Non-O based on being over 50 there is no insurance requirement when you apply for an extension inside Thailand. Now you just need to see if your embassy in Bangkok will provide you with an income affidavit and if your thai immigration office will accept it, or whether you need to find 800k baht to put in your bank. Remember the latter needs to be there 2 full calendar months before applying for an extension

If you didn’t have to prove a mandatory health insurance when you applied for your visa, there’s no mandatory health insurance requirement for the coming extension of stays either.

And

@Dianne *********

I remember you applied in Malaysia where there in fact is a health insurance requirement also for the Non O visa. Thailand don’t have such requirement for your coming extensions in the future.

Depends if it is an extension on a non-o or a non-oa I believe.

Not if you are extending a Non-O Visa