I think it depends on where (or even if) you draw the line on free speech when it comes to verbal harassment.

Here's a quick test. Let's say Bob goes for a walk, but Bob's neighbor, John, is a complete dick and whenever Bob leaves his house to go for a walk, John follows him and just hurls non-stop insults at Bob.

Should John's behavior be illegal? You could argue that as long as he's not trespassing anywhere, and he has the right to free speech, then John should be legally allowed to do this. In that case, it makes no sense to make deadnaming a criminal offense. There's no discussion to be had, as we've decided that asshole-ish speech is not illegal.

If you say no, it's verbal harassment, and should be illegal, then you're saying free speech does have limits, and we just gotta decide if deadnaming is bad enough to be illegal.

This kind of behaviour would most likely be prosecuted as harassment/stalking instead of hate speech/libel.

So unless you follow someone around and harass them - just saying someone's so-called "deadname" shouldn't be illegal.

John's behavior already is illegal in most Western jurisdictions. Your example does not demonstrate why deadnaming in particular needs to be a criminal offense.