> a deterministic, mostly terse, language

Ah, like some sort of "programming language"? A weird idea, but it could work!

It's called C. With all the undefined behavior it's mostly deterministic!

Kinda, more output tokens usually correlates with better benchmark scores. Ideally LLMs would keep that in their thinking section, then draft a response (what they write currently), then output something short. It'd consume even more tokens, but we wouldn't see that text

Terse and unambiguous seem to be at odds with each other. You might want to look into Lojban and similar constructions.

If such a language existed, it would surely take a human years of study to become proficient at it.

A lot of users are subsidized (if you're in doubt, consider the wealth of free users).

It's a shotgun approach to answering questions. If it's terse it might only mention 1 of 10 facts it could provide, and that might not be the one you're looking for. So they just say a fuck ton of words and are more likely to meet the needs of everyone asking your question. If they miss it you'll prompt it again and they have to perform a second pass of inference, which costs them more money.

Lisp

It’s not.