I don't get why this is downvoted. Even if the second sentence is I correct, the first one is correct.

I mistakenly assumed the "+" thing is built into the email standard.

Fastmail supports it too and you can configure postfix and opensmtpd to support it as well.

More than Gmail supports this.

I can confirm it works on MailInaBox. So it should probably work on any installation that uses Postfix and/or Dovecot.

Most providers support it afaik.

Additionally, gmail normalize all periods (.) to the mail address without periods.

So [email protected] [email protected] both get delivered to [email protected]

iCloud Mail supports it too