Her entire Twitter account is transphobia.
All of it. Calling trans women fetishists, claiming trans guys are all autistic mentally ill survivors (!!), taking the piss out of individual trans and nb people, pushing the 'trans away the gay' lie...only someone who is themselves transphobic refuses to see it.
She's also made some absolutely hateful, repulsive comments about asexual people.
Dismissing her behaviour as 'standing up for women' is deeply disingenuous. That doesn't explain why she reposted a thoughtful article about how a nb parent defined themself with nasty, disparaging comments, to give just one example.
All from a woman who employed a domestic abuser, refused to fire him, defended her decision and ignored the DV groups who asked her to reconsider.
Her attitude towards trans people is extremely consequential when they are suffering very real violence - particularly in schools.
She has reposted Islamophobic content, replied approvingly to obvious fascists and denied that trans people were killed during the Holocaust. She hasn't distanced herself from KJK or any other Islamophobic, racist anti-trans people.
She even defended that hateful, racist danger Linehan.
I'm aware asking 'what JKR has said that's so transphobic' is rarely done sincerely, but just in case: read her own words. Read, if you can stomach it, what she and her cronies (inc that worrying bloke who runs a trans woman parody account) say about trans women's bodies.
Read what she said about Imane Khelif, a young Muslim woman in whose country it's illegal to be trans. Look at how she publicly treated her, based upon little more than rumours and Khelif's appearance.
Khelif is a massive inspiration and fantastic female role-model for young girls in Algeria. They love her for that.
JKR's targeting of a brown Muslim woman has been noted by people normally uninterested in her anti-trans comments. She isn't a doctor, or on the Olympic Committee. It's horrific what she's put that young woman (who's lost plenty of matches to other cis women btw) and others through.
The evidence is all there. Bigots don't get to say they aren't bigots, that's for their targets to name.