If I'm not mistaken, it's a Gamil specific feature. Any combination of words or numbers between + and @ is seen as not part of your email address but can be used by filter received emails.
It's not gmail specific, it's standardized as a "sub-address" and not part of the username of the email. smtp has worked like this since the 90s at least.
Do you have a cite for this? AFAIK "plus addressing" always been an optional implementation done by certain email providers themselves, but wasn't an actual part of the standards.
See this link for more information: https://gmail.googleblog.com/2008/03/2-hidden-ways-to-get-mo...