You don't always get an IFS report even if someone is punished.
Also, simply being toxic in one game is generally not enough to warrant a punishment. Unless they use zero-tolerance words (automatic 14 day ban) or are really toxic, it's generally not enough for even a restrict.
Severity x consistency is what Riot uses. If it's inconsistent enough, you could (theoretically) flame the everlasting shit out of someone for 20 minutes straight and not get punished. You shouldn't do that, but that's probably how it works (please do not test it). You could also, in theory, do a very low level of flaming every single game, and if it's low-level enough you also won't get punished (again, please do not test that). This is because everyone has shit days, and Riot recognizes that. Sometimes people just tilt and flame others, and you shouldn't be punished if you're otherwise a good person (within reason, of course!).
Either way, no IFS report=!No punishment. And you cannot verify that they weren't punished for toxicity, since you have no way of seeing whether or not someone got chat restricted.