As much as I dislike Parler, I very much so hope that ISPs don't ban it. Imagine if ISPs get to block whatever website they deem dangerous. That cannot have a good ending.
Somehow ISPs are going to use this as an argument against https etc.
This might be an interesting argument for municipal broadband which occurred to me last night: in that situation the ISP is in fact bound by the First Amendment, which limits its ability to do that sort of thing quite a bit...