Where will it be written to and how is it guaranteed to be picked up by any further invocations of the tools?

Yeah, given go's history with breaking changes (and the habit of OKR-chasing managers and Senior Staff to make Number Go Up to look good on the promo packet), I definitely would not trust any opt-out mechanism to not receive operationally-significant changes in the next five years.

As far as I known go never changed any flags since release ( 2012 ), is it good enough for you?