Maybe what OP was getting at is more like a "transgenderism is a mental illness" type of heretical statement
Either way, some ideas are "heretical" for a reason: they lead to worse outcomes. You could argue under the same "trans are mentally ill" model that gay people are too. But we don't, because letting gay people live their lives (or more accurately, not oppressing them) leads to better outcomes. That's what it's all about. And it's the same with trans people - if they're accepted and allowed to present how they want, outcomes are better.
My position is mainly that unethical things shouldn't be advocated for. I think there's no good argument for stopping trans people for living the lives they want to live, and that people will rightly criticise you for being opposed to them exercising their freedoms. It's not that these things "can't be said" but don't be surprised if people criticise you for it. I think people calling this heresy are being melodramatic - being ratio'd on twitter is not the same thing as being burned alive at the stake.