Apparently the Stack Exchange terms changed two weeks ago. According to this the change went live on the second of May, and e-mails should have been sent "in the days ahead".
Well, it's now two weeks later. The only e-mail I received from Stack Exchange recently is the one advertising Stack Overflow for Teams, sent on the 8th of May. There's also been no obvious sign on the home page or in my inbox. The first I heard was a few minutes ago when I spotted this post on Reddit.
WRT the wording "you must notify us in writing within thirty (30) days of the date that you first accept these Public Network Terms (unless a longer period is required by applicable law)", I also point out that even now that I'm aware of your new terms and have read some of them, that doesn't mean I've agreed to anything.
In principle I believe the idea of mandatory arbitration is bad. Wording the opt-out clause to require a snail-mail opt-out where the vast majority of users have NEVER contacted you any way about anything except electronically is bad. I'm aware of the electronic opt-out, but the actual TOS still appears to contradict that - when the legal document you want people to agree to make claims that contradict what you claim elsewhere, that is bad. And so far I've been unable to discover what counts as "agreeing" to the new TOS, especially in the case of people who are completely unaware of them because they haven't been notified, either by e-mail or by an obvious message on the home page. Pretty bad again, plus I searched the TOS for "without notice" - that all-too-common cop-out isn't there unless you've worded it a non-standard way. So the fact that it's not there is good, but I still note that I haven't received notice, though you had more than one way to easily tell me.
I'm not certain I have any practical reason to care about the arbitration clause in this case, but I'd need time to think about that. Time that appears to have been maybe wasted because of the lack of notice, further time that will be wasted tracking down information scattered around various different Meta questions (though ultimately I assume it's the TOS that are binding, not the questions and answers that appear to contradict the TOS).