I mean I get it might be because of being afraid of it being used to train LLMs. But I doubt that it would work, either because they won't be used regardless or because of how federation works, i.e. literally it'd be more efficient if all of the known network instances' operators somehow agreed to include/Lemmy, kbin and all of the microblogging platforms that can federate with Lemmy shipped a robots.txt that blocks known AI crawlers. Probably what would be more useful would be something that e.g. Akkoma and some other AP implementers offer, i.e. message autodeletion.

Also terrible if you want to retain any anonymity even if more people did it, because of stylometry.