Is there a reason why this mode https://github.com/optick/codemirror-mode-elixir not included in modes by default?
Yes, there is. We’re moving away from bundling modes in the main library distribution, so recently written modes will be distributed separately.
I absolutely get the idea. Maintaining all these languages is hard.
But then developers of various software that embeds CodeMirror do not include the additional languages either. As a result users don’t get them at all.
For example, Atlassian doesn’t give users Elixir:
I think that it would have been much better to have Elixir in core.
Of course it’s your call.