> Can you cite an example of a big, well-known, etc service that enforces a 'you can't change your email address' rule?

Philips Hue.

I've run into others, but that one sticks out because it's the only account I still have using my old email, since creating a new account would also involve re-pairing all of my Hue products.

https://www.coles.com.au/ (the 2nd largest supermarket chain in Australia) and https://makerworld.com/ are two I've come across.

McDonald's. I wish I was kidding. https://www.mcdonalds.com/us/en-us/faq/can-i-change-the-emai...

My hospital.

Yeah, and they insist on using it for MFA.

Actually the tech support did change what I use to sign in, but the patient record/profile is separate, and not user-serviceable.

I tried twice to change it, but they were able to send me on epic bureaucratic fools errands; I had to give up!