/usr/share/nvim/runtime/plugin/spellfile.vim is in neovim-runtime 0.2.2-3.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 3 4 5 6 7 8 | " Vim plugin for downloading spell files
if exists("loaded_spellfile_plugin") || &cp || exists("#SpellFileMissing")
finish
endif
let loaded_spellfile_plugin = 1
autocmd SpellFileMissing * call spellfile#LoadFile(expand('<amatch>'))
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