You have to give credit to whomever came up with it, its a cheap but effective linguistic kill shot. No doubt it was engineered to be divisive, and whomever is wielding it is unlikely to be reasoned with.
Who is saying that people should be harmed? Seriously. Whoever you have in mind, if they're actually advocating violence against people then I'm in agreement with you. That's not cool and they shouldn't be doing that.
Are you referring to calling a trans person by their biological sex?
When a doctor is speaking with a trans person about their body, it's an appropriate time to speak about their biological sex. That is certainly not hate speech.
When a stranger is yelling at a trans person about their biological sex, it's done to inflict emotional harm on the trans person. They transitioned away from that gender to reduce harm (that is the definition of a trans person), and the stranger is intentionally trying to bring that harm back.
Imagine shouting at a woman about her "biological breast size" because she is wearing a push-up bra, or had surgery to enlarge or reduce her breast size. Would that seem like a normal, harm-free way to interact with another person you don't know? Obviously not.
Depends on the context. People have been suspended from Twitter from saying things like "transwomen are actually men so stop letting them compete in women's sports, it's not safe or fair for actual women", which isn't hate speech, doesn't target an individual, and is a legitimate political opinion, so in theory should be fine. Yet bans are handed out anyway.