I tried to have an open mind about that letter, however tying what they're careful to not call cancel culture, as a right wing ideal is a steaming pile of crap that I couldn't overcome. I do find it interesting that we can all find some common ground on pieces of this and can completely disagree on other parts. In some ways I'm configured with viewpoints on this issue, in gE anyway, as I've grown very weary over the zero sum game all aspects of life has become. I don't feel the same way about Harper's editorial board and the people who signed that letter. They are not friends talking in their living room, they are people in position of influence and are using that influence to do exactly what they are speaking out about, and that is not allowing people to be able to freely express themselves.
I don't think cancel culture is a thing, not unless you acknowledge that people for the as long as words have existed, have either fairly or unfairly had repercussions for the things they say and feel.
It is completely self serving for these people to complain about words having consequences, given almost all of them make a living or are able to enhance their profile by speaking publicly. I guess it's no surprise that these people, largely Gen Xers are attempting to create a society where your words mean less. No one told JK Rowling to poke at trans people, more than once. While most American's could say the things she's said without repercussion, some couldn't, I sure as hell couldn't, not that I'd want to.
The best way to not get cancelled is to not have to publicly broadcast all of your thoughts, I don't know why that's so difficult, we've all spent everyday of our lives doing just that.