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.

Niche is fair. Basically, I'm trying to describe "not mainstream".

By this metric, Ruby, Swift, Go, Rust, Kotlin, and Typescript are all obscure. Some of these are even more obscure than 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.