That was about five years ago, I imagine it might have been more difficult in the past, say 10+ years ago.
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.
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!
I wouldn't say it is easier than expected though, logging in, changing your email, confirming the change, and then making sure the password manager was updated was a bit of work.
I had all the important stuff done in a week.