The number of websites that prevent me from doing this, because somebody wrote a shitty regex to invalidate most punctuation in an email address, is infuriatingly high.
Not really. I'm assuming the reason you'd share an email is so that you only need to be logged into one account in your mail client.
Easier to remember than a username because it's guaranteed to not be taken, so you can use the same email everywhere: email+name@provider.tld. Where as "alice" probably is taken.
Also should work with any email provider, it's part of the standard.
The only mail provider I ever heard of this working with is Google. And if you already support multiple usernames per email address, why not support using the same username for different email addresses? It's not like leaving it blank couldn't be valid, too. After all, it's the combination of email + username that is the actual DB key, just like it is with email+name@provider.tld
If the intent is to allow two people to have independent accounts even while using an email they both control, offloading that to the email protocol seems broken to me. It's the exact same email address from the perspective of security. Anything coming after the plus sign should be ignored for the DB key, but kept around for sending emails, so it can still be used for filtering those emails (for convenience, not security). So they could sign up either as
alice <couple@notgoogle.com>
bob <couple@notgoogle.com>
or as
alice <couple+alice@notgoogle.com>
bob <couple+bob@notgoogle.com>
but that difference should only ever matter for their email filtering, not for identifying them.
