> Will Github allow people to use other $EDITORs?

Yes, just clone? Most people who want other editors probably don't want browser-based ones.

I'm not sure if the VSCodeVim or VSCodeNeovim extensions work on the online version yet, but I'm sure some VSCode/GitHub engineers are on the case if not.

EDIT: VSCodeVim (the main "Vim" extension) does "work" on github.dev but the escape key doesn't, so as soon as you press "i" you're stuck in insert mode forever. https://github.com/VSCodeVim/Vim/issues/7005