The ugly trick(not original to twitter, I learned this one good and hard in school, grades 1-8) is that while threats are an option for ruining someone’s day, sufficiently dogged petty harassment works pretty well and involves few, if any, acts that rise to the level of ‘true threat’ or similar standards.
There certainly are people on the internet who are making threats that have never been protected speech and have always been legally problematic, and that presents an option for dealing with them; but you needn’t break the law to be a baying pack of insufferable assholes.
Of course, ‘protected speech’ is largely irrelevant in the context of privately operated messaging platforms; and it is substantially on them to do something about the fact that enough of their users are terrible people that their services are functionally unusable for many potential customers.
