Not only are they absurdly obsessed with it, and offended for the sake of being offended... I can't believe I'm going to defend amber here, especially for something like this because I usually fall on the OTT social liberal side, but... Amber did an incredibly shitty thing when she lied about the abuse and rape. However, when you have a shared history with someone, I don't think the one who changes gets the right to own the narrative about someone's past with them. Amber was in a relationship with a female, and yes she was intolerant when she learned her partner was trans, and that's shitty, her partner hadn't yet begun to transition or use a male pronoun. But Amber was in a relationship with [insert former name, I just can't remember it, I thought it was just casey w a different spelling], she wasn't in a relationship with Kasey the male. I think she should still have the right to refer to her ex girlfriend as she/her when talking about their past, because it was her girlfriend, not her boyfriend. Casey transitioned, but that doesn't mean that Amber should have to rewrite her history to change it to a straight relationship. Now referring to the person in the present, she should use the appropriate pronoun.
For example, If I had a boyfriend named John, who transitioned after we broke up and is now Jane, I wasn't in a lesbian relationship with Jane. I feel like it should be my right to talk about my relationship with John, even though John is now Jane. Because I still own the right to my side of the relationship, and it wasn't a relationship with a woman. And then obviously code switch to the correct pronoun to talk about Jane's present life.
I don't know if this makes me an asshole or not, but yeah, I feel like amber gets a pass for using Casey's gender pronoun as it was while they were together when recounting their relationship. Where she really fucked up was saying that Casey is now Kasey (or whatever the original name was) and talking about the transition. It wasn't relevant to her false story about rape. She could have just talked about a former girlfriend and left it at that.