Note:
(a) And Then There Were None https://en.wikipedia.org/wiki/And_Then_There_Were_None
(b)
(i) lyrics of Ten Little Niggers, by Frank Green in 1869. https://www.parkkinen.org/ten_little_niggers.html
(ii) Ten Little Indians https://en.wikipedia.org/wiki/Ten_Little_Indians
(section 2 Minstrel song: 1968; section 3 Derivative songs and books: "It is generally thought that this song was adapted, possibly by Frank J Green in 1869, as 'Ten Little Niggers,' though it is possible that the influence was the other way around, with 'Ten Little Niggers' being a close reflection of the text that became "Ten Little Indians". Either way, 'Ten Little Niggers' became a standard of the blackface minstrel shows. It was sung by Christy's Minstrels and became widely known in Europe, where it was used by Agatha Christie in her 1939 novel of the same name, about ten killings on a remote island")
(c) none (n; "singular or plural in construction") https://www.merriam-webster.com/dictionary/none
That is why the title of the song uses "were" whereas my habit is always singular: I would say "And then there WAS none."