Eclipse has had an integrated spell-checker, which I believe is on by default for most file types, for like approximately forever. Now maybe everybody turns it off, but I gotta imagine there are some people who like it and keep it on.
Emacs has the ability to do spellcheck inline, both as a run through the buffer (old-school style) and as an as-you-type live feature. That said, I do most of my coding in JetBrains IDEs these days.
For Vim/Neovim users, there is one built in that is pretty good, and once you've added frequent custom words to the dictionary it is great. You can turn it on with `:set spell` or off with `:set nospell`. Add custom words by pressing `zg` on the target word:
I have this in my vimrc file so it's on by default for certain file types:
" Turn on spellcheck for certain filetypes and word completion.
" words can be added to the dict by pressing 'zg' with cursor on word.
autocmd Filetype markdown setlocal spell
autocmd Filetype gitcommit setlocal spell
set complete+=kspell
" Don't highlight in red an underscore (_) in markdown
" https://vi.stackexchange.com/q/18471/17441
autocmd Filetype markdown syn match markdownIgnore "\v\w_\w"
Custom additions to the dictionary will go to a simple text file (one word per line) in `~/.vim/spell/en.utf-8.add` (depending on your settings) where it is easy to edit or backup.
You can also add it directly when using it. Move the cursor to the word and (I forget the command...) can add it as a rare word, good word or bad word.
They weren't integrated into programming-oriented editors, and it would have been unusual to run them against code.