There were a few obscure online stores that I can't remember but the ones I do remember I don't want to link for privacy reasons. One was a local water utility company and the other was my university which offered academic email address forwarding for alumni. It was silly as a Protonmail address also didn't work for one of them.

Anecdotal, but I've seen lazy/clueless developers in some companies who allow only a .com email address to be provided. And this was a few years ago. I don't recall the name. I'm not going to do any business with such services.

Nearly 25 years running my own domain email and I have never encountered this either. (Currently using GAFYD for email, but at times it's been virtual hosted, as well as home hosted on a Pentium Pro and dial-up)

Same here, I have been running my own domain for 20 years. I have gotten the 'reverse' though... trying to register somewhere using a gmail throwaway account and getting the message that they do not accept free service emails.