You can do spell checking with just vi and a program called aspell. Most UNIX like OS's should have aspell in their repos, otherwise you can get it from aspell.net.
Once it is installed you by running the following commands in command-mode:
:w!<CR>
:!aspell check %<CR>
:e! %<CR>
This saves your file, runs an interactive spellcheck, then loads the modified file into the current buffer.
Now you don't have to install all of vim or neovim just to get spellcheck.