If you have not been able to find via an online search what Starmer has or hasn't said, I don't think I will be able to do so either.
Rosie Duffield has said things that are widely considered as transphobic; I believe she also is anti-LGBT rights; this is pretty extreme especially in today's climate, whatever you think of that. There is no excuse for threats of violence; however, what do you want Starmer to apologise for? Even if he should, I don't think he is likely to - again due to the divisive nature of the debate.
Anyone coming in strong on one side or the other, along with stating such a view, is going to lose a lot of votes in one camp or the other. I'm not saying this is right or wrong, but just the likelihood of what is happening in politics pre-election. Starmer has already said his piece in November apparently. He may well get questionned further, but just in my opinion, I think it is unlikely that any of the main political leaders are going to give you the answer that you want.
Don't shoot the messenger though! That's just what I think is likely and I could be wrong.