I believe it was The Hague convention that “outlawed” deforming projectiles, as the damage that they imparted was considered inhumane. Gotta remember at that time militaries we’re still using lead RN rounds. There’s a reason the Civil War was the bloodiest conflict in US history. Those big, heavy, soft projectiles destroyed everything they hit inside a person. That’s why FMJ became the military standard, because anything else is technically a war crime under The Hague.
That said, all is fair in love and war, and that rule has largely been broken and/or unenforced. Hell, when I was in Afghanistan our SDMs had M14s with hollow points.
I’ve been curious about FMJ 5.56 for nutria, simply because the ammo is plentiful and those critters are invasive and considered “unprotected non-game” here, which pretty much means F&W doesn’t regulate them at all. I hear they taste pretty good, too. Nevertheless, it’s only worth shooting something if it’ll actually kill it quick and clean.