Han är en tidigare Vala.
Som du säger är Maia och Valar båda bara klassificeringar av Ainur. Den fullständiga passagen du citerar är följande:
The Great among these spirits [the Ainur who came to Arda] the Elves name the Valar, the Powers of Arda, and Men have often called them gods. The Lords of the Valar are seven; and the Valier, the Queens of the Valar, are seven also. These were their names in the Elvish tongue as it was spoken in Valinor, though they have other names in the speech of the Elves in Middle-earth, and their names among Men are manifold. The names of the Lords in due order are: Manwë, Ulmo, Aulë, Oromë, Mandos, Lórien, and Tulkas; and the names of the Queens are: Varda, Yavanna, Nienna, Estë, Vairë, Vána, and Nessa. Melkor is counted no longer among the Valar, and his name is not spoken upon Earth.
(Valaquenta)
Notera passets tid, som i sammanhanget är en bok av myt och historia. Inte längre är Melkor räknat bland Valaren, och hans namn är inte talat på jorden. Detta kan bara referera till nuvarande tid (eller relativt aktuella tider), men säkert innan Feanor bytt namn till honom.
Senare i slutet av Valaquenta:
Last of all is set the name of Melkor, He who arises in Might But that name he has forfeited; and the Noldor, who among the Elves suffered most from his malice, will not utter it, and they name him Morgoth, the Dark Enemy of the World. Great might was given to him by Ilúvatar, and he was coeval with Manwë. In the powers and knowledge of all the other Valar he had part, but he turned them to evil purposes, and squandered his strength in violence and tyranny. For he coveted Arda and all that was in it, desiring the kingship of Manwë and dominion over the realms of his peers.
From splendour he fell through arrogance to contempt for all things save himself, a spirit wasteful and pitiless. Understanding he turned to subtlety in perverting to his own will all that he would use, until he became a liar without shame. He began with the desire of Light, but when he could not possess it for himself alone, he descended through fire and wrath into a great burning, down into Darkness. And darkness he used most in his evil works upon Arda, and filled it with fear for all living things.
Valaquenta säger Elves som heter Valar (Valarens medlemskap definieras därför av älvorna), som helt enkelt var "Stora" bland Ainur som gick in i Arda. Så är frågan en fråga om när älven skulle ha ansett att Melkor inte längre var en av de stora.
Vi vet inte vilka berättelser den andra Ainur kunde ha sagt till Eldar of Melkor. Om de sa någonting, föreställer jag mig att de lämnade det på att han var en av de största bland dem, men ändå upproriska. Hur som helst, för en tid ansåg älvorna Melkor som en bra kille.
Then Manwë granted him pardon; but the Valar would not yet suffer him to depart beyond their sight and vigilance, and he was constrained to dwell within the gates of Valmar. But fair-seeming were all the words and deeds of Melkor in that time, and both the Valar and the Eldar had profit from his aid and counsel, if they sought it; and therefore in a while he was given leave to go freely about the land, and it seemed to Manwë that the evil of Melkor was cured. [...] Therefore all the more did he feign love for [the Elves] and seek their friendship, and he offered them the service of his lore and labour in any great deed that they would do. The Vanyar indeed held him in suspicion, for they dwelt in the light of the Trees and were content; and to the Teleri he gave small heed, thinking them of little worth, tools too weak for his designs. But the Noldor took delight in the hidden knowledge that he could reveal to them; and some hearkened to words that it would have been better for mem never to have heard.
(Of Feanor och Unchaining of Melkor)
Allt slutade ganska snabbt.
Then Fëanor rose, and lifting up his hand before Manwë he cursed Melkor, naming him Morgoth, the Black Foe of the World; and by that name only was he known to the Eldar ever after.
(av Flight of the Noldor)
Så, ser tillbaka till ditt citat, var hans namn, vid denna tidpunkt "inte längre talat på jorden", åtminstone av Eldar, som påstås ha skrivit de historier som våra Hobbits bygger The Silmarillion på . Jag tycker att det är logiskt att Eldar inte längre hade ansett Morgoth en av Valar.
Men du kan göra argumentet att Morgoth fortfarande var en av "Arbetsmakterna", Black Foe of the World, eller inte. I det fallet skulle jag säga att han definitivt upphörde att numreras bland Valaren före slutet av krigets krig:
The war was successful, and ruin was limited to the small (if beautiful) region of Beleriand. Morgoth was thus actually made captive in physical form,9 and in that form taken as a mere criminal to Aman and delivered to Namo Mandos as judge — and executioner. He was judged, and eventually taken out of the Blessed Realm and executed: that is killed like one of the Incarnates.
(Myter Transformed)
Att bli otrevligt fångad, dödad och utvisad från Arda skulle verkligen tycka att diskvalificera dig som en av Ardas stora styrkor - även om hans ondskan bodde på.