I've only seen "esoteric" refer to programming languages that are deliberately constructed to be obscure or unusual, usually in ways that make them unusable in some practical sense. Your Brainfucks or Malbolges, say. I've never heard of anyone applying that term to Lisp or especially F#. They might be unpopular, but they're usable, practical languages with good implementations.
Obscure is a good choice. I’d probably use niche. Honestly, esoteric could be a good word to describe lisp, except for the fact that it seems to already exist as a term for a category that wouldn’t include lisp.
Correct. I know plenty of enterprises that do not touch any of those, mostly because of their fear of not being able to find devs familiar with those languages.
As discussed elsewhere, it's possible niche might be a better term to describe what I'm talking about here than obscure.
