You used to be able to adjust your email address to check. For example if you email was bill@gmail.com, you could sign up for HN with bill+hackernews@gmail.com. Gmail ignores the part after the + sign. Therefore if you noticed emails coming to that address, you would know that HN sold their list. However, I've found that most forms reject that as a non-valid email address now.
People have had their Google accounts suspended (especially if it's associated to say, a YouTube or AdWords account), and now you have no access to your address anymore, and Googles free tier support won't help you.
With domains there's an ICANN process to get your domain back if it gets hijacked.
(a) Using your own domain doesn't require using your own mail server, you can point your mx server at Google apps if you're comfortable or your registrar probably supports mail forward if you don't want to pay for G Suite.
(B) it means you can keep your email address if you ever leave Gmail.
Don't any half decent sites strip that out anyways? Some ecommerce sites have actually failed to accept that string, inadvertently thinking it's invalid. The rest, or any marketing CMS, would simply remove it.
Many sites reject valid email addresses. One character it is common for forms to reject is a "+" in the left hand side of an email address. The email RFCs allow this character, so denying it is bogus. Nevertheless, they do.
