It is, but many services will (incorrectly) prevent an address with a `+` in it from ever making it to their database/whatever.
So the fact that the MTA will route it is irrelevant if it never makes it to the MTA in the first place.
So the fact that the MTA will route it is irrelevant if it never makes it to the MTA in the first place.
+ as a magic character to effect routing isn't part of the standard. Mail servers are free to route addresses to mailboxes in whatever manner they see fit. That + can appear as a character in an address is part of the standard, just not the behavior of it; a server that treats a+1@ and a+2@ as distinct emails is conforming, and from a sending side, you cannot know if a+1@ and a+2@ will end up in the same mailbox.
(But you're absolutely right that too many sites fail to parse email addresses. Or rather, they over-parse.)