That's still not an argument to introduce ai.txt, because everything a hypothetical ai.txt could ever do is already done just as good (or not) by the robots.txt we have. If a training data crawler ignores robots.txt it won't bother checking for an ai.txt either.

And if you feel like rolling out the "welcome friend!" doormat to a particular training data crawler, you are free to dedicate as detailed a robots.txt block as you like to its user agent header of choice. No new conventions needed, everything is already on place.