Terf stands for "Trans exclusionary Radical feminist", which is a type of feminist that pushes for women's rights, but doesn't support transgender rights, and thinks MtF transgender people don't count as women.
A lot of people have boycotted Hogwarts legacy because of her political views. Personally, I think it's a bit extreme to boycott a great game made by a studio and developers that have nothing to do with her views just because she gets royalties on it, but that's a matter of personal opinion
Not any reason though, the case didn't change any of the protected classes like sex, religion, or sexual orientation. It just made it so a company can choose what "expressive work" they want to do, especially websites. So it's legal to say you don't want to make someone a custom website if you disagree with the contents of the website (ie a website that supports gay marriage), but it's still illegal to refuse to make someone a website because the customer is gay. You can choose what you make, but you can't choose who you sell it to