To put this topic firmly back on track, as it is something of importance to myself.
Yes, Lucifer is a deity. In my own Tradition, Lucifer is one of the ruling powers, next to both his mother Hecate, and his brother Belial. I can't speak for others - i can only speak of what i myself know and have experienced.
All mythological origins of the name when considered in this context are rescinded - instead direct working with him and the other Spirits that know of him reveals his true multiple and many natures, with both him and his Brother Belial being excellent shape changers.
He is an entity which has appeared across multiple cultures, in multiple names, and is most assuredly one of the better allies anyone who practices any form of magick can have. Anyone who is unsure of this can simply test it for themselves. He is usually a very vocal God... using multiple ways to delivers messages of his existence and presence.
He is easy to contact at least for myself - all you need is the Sigil of the Temple of Pyramids, his usual sigil (at end of post) written in a consecrated ink on virgin paper or parchment. Simply brew some coffee, enhance the paper with it, let it dry, write on the sigil, let it dry, pass over sandalwood incense, spit alcohol from the mouth of the sigil (lightly), and then blood the sigil from a cut in the left hand or finger. Meditate on this sigil and he will assuredly come in some form and communicate with you. His power is unbelievably intense, and ranges from intensely cold to blazingly hot depending on the form he wishes to take. He prefers appearing with a red or even gold aura, but sometimes he appears as a blue / light green aura, mirroring his counterparts.
It gets me quite mad people simply describing him as an 'icon of rebellion' or other such nebulous terms, frankly bullshit terms to lend agency to their teen angst cults without ever having tried to contact him and work with him firsthand to see his real nature, and then go on to also deny him any recognition of his external agency.
I would also point out a deity is not what you want it to be - a deity is either real and existent in some form as a powerful identified spirit to venerate (whether you believe that is simply an archetype or a solid external being capable of external materialization) or it isnt a deity. Too many people believe you can force whatever concept into whatever form you like to make it work, without realising that has profound issues lying behind it.
Its this mindset and approach that causes alot of problems for occultists these days. Syncreticism without fucking things up is exceptionally hard to do in a manner which works off the bat and its best done in a natural and carefully considered fashion.
Sorry if this has come across a little ranty. I believe its important to put two or three several counter points out there.