Possibly a good idea for a writer who wants to write Arthurian stories is to start reading them from the earliest writings onward and note which plot points are out of copyright.
You could start by reading about the possibly historical Arthur in the Historia Brittonum and the Annales Cambriae.
Then try reading early Welsh Arthurian stories and poems like "Culwich and Olwen" and "the Dream of Rhonaby" and the Arthurian Triads.
Then read Geoffrey of Monmouth's History of the Kings of Britain which depicted an allegedly historial arthur and made him famous through out Europe, and his successors like Wace, who introduced the Round table, and Layamon.
Then read medieval romances, basically historical novels set in the era of Arthur, by writers like Chretein de Troyes who introduced Lancelot and the Holy Grail into Arthurian Stories, and Robert de Boron who introduced the Sword in the Stone, and so on.
And you should read a lot of 19th, 20th, and 21st century Arthurian novels, or read various accounts of modern Arthurian literature which descirbe which novels added new plot elements and which novels later used them. You have little hope that the heirs of such recent writers would be unaware of being their heirs or would ignore any copyright infringements.
And you should watch a lot of Arthurian movies or otherwise learn about their plots.