In what sense do you think this isn’t following the email standard? The plus sign is a valid character in the local part, and the standard doesn’t say how it should be interpreted (it could be a significant part of the name; it’s not proper to strip it out) or preclude multiple addresses from delivering to the same mailbox.

Unfortunately the feature is too well-known, and the mapping from the tagged address to the plain address is too transparent. Spammers will just remove the label. You need either a custom domain so you can use a different separator (‘+’ is the default but you can generally choose something else for your own server) or a way to generate random, opaque temporary addresses.

If you want to talk about non-compliant address handing, aside from not accepting valid addresses, the one that always bothers me is sites that capitalize or lowercase the local part of the address. Domain names are not case-sensitive, but the local part is. Changing the case could result in non-delivery or delivery to the wrong mailbox. Most servers are case-insensitive but senders shouldn’t assume that is always true.