Yahoo Mail doesn't use the plus, but they allow you to use another text as a prefix, which means spammers can't know the real email address behind the aliases.

Yahoo Mail doesn't use the plus, but they allow you to use another text as a prefix, which means spammers can't know the real email address behind the aliases. For example, if my real address is myname@, I can set my prefix to be myothername and all my aliases will be myothername-something@. myothername@ won't be an address, so spammers can't send anything to it, and aliases have to be registered to be able to send emails to them. It's slightly more annoying than the Gmail plus system, but way more secure.