I would prefer a more generic "license.txt" i.e. a standard sanctioned way of telling the User Agent that the resource under a certain location is provided with a specific license. Maybe a picture is public domain, maybe is copyrighted but freely distributable, or maybe it is but you cannot train AI on it. Same for code, text, articles etc. The difficult part would be to make it formal enough so that it can easily consumed by robots.
With the current situation you either assume that everything is not usable, or you just not care and crawl everything that you can reach.