That's why I learned vi initially years ago, because I was told it was the only editor that would always be available anywhere you went. I only ever use it in ssh sessions these days, but I still feel like learning how to use it has paid off many times.

I was playing with Haiku recently and was surprised that vi or vim wasn't included in its base install. Granted, Haiku (like BeOS before it) doesn't ever claim to be a Unix-like or POSIX-compliant system, but I found the CLI environment to be very Unix-like overall. I guess it's a testament to the ubiquity of vi that it never occurred to me that it wouldn't be there. (If anyone's curious, emacs is also absent but nano is there.)