This is one big batch email we sent out to everyone who, in the logs from the last two (and a bit) months, had a validation success from our primary perspective but a validation failure from one of our secondary perspective.
There's a separate email going out on an ongoing basis for ACMEv1 deprecation. Every two weeks we are sending an email to everyone who issued an ACMEv1 certificate in the prior two weeks. The subject is "Update your client software to continue using Let's Encrypt".
I'm very sorry to contribute to your high support burden and high blood pressure! We definitely try to minimize disruption as much as we can, and in this specific instance we had a tough choice to make: contact everyone who might be affected by multi-perspective validation, even though some would have no problems, or keep support burdens low by not emailing people and letting them ask for help as they ran into problems. We opted for the notification path, and I still think it's the right choice, but it's really valuable to hear from maintainers about the impacts it has had, so thank you.
I do think it's worth reiterating that since the beginning, we have emphasized that part of "set it and forget it" is keeping your software up to date. Auto-updating software is, in my opinion, the next step in ensuring that people's sites continue to work smoothly without much intervention.