It will be hard to satisfy your demand for proof, because there's no good way to search "tweets you've seen", but I have seen it happen.
Person A starts a thread.
Person B who A doesn't know says something dehumanizing to person A.
Person A says "fuck off and die".
Person A gets shadowbanned for a week.
There was even a time when, due to their bad automated processes, you could get shadowbanned for using the word "queer" in a positive sense.
(Hilariously, Twitter's position on shadowbanning is that they do no such thing, they just hide your tweets from everyone's timeline, which is exactly what everyone means when they say "shadowban".)