Apple giveth, and Apple taketh away. And Apple hath taken away faxing.
Apple has never been afraid to abandon technologies that strike it as obsolete, and faxing is one of them. Heck, Mavericks can’t even fax if you have one of Apple’s own USB fax modems.
If you still need to fax, you do have a couple of options. First, you can sign up for a service like eFax or Faxaway, which let you send and receive faxes online. Second, you’re welcome to buy a non-Apple fax modem, some of which work fine. Whatever you buy comes with its own software and instructions.