What is the most niche programming language that is still written?

I would not be surprised if TAL is still used for https://www.hpe.com/us/en/servers/nonstop.html boxen.

Similarly, https://www.unisys.com/client-education/clearpath-forward-li... probably still uses its bespoke languages ( https://en.wikipedia.org/wiki/Burroughs_Large_Systems#ALGOL ) in places.

Do you have any interest in academic languages?

After a half a century or so without, BCPL recently got floats (for a flight simulator): https://www.cl.cam.ac.uk/~mr10/BCPL.html

I prefer the author's MCPL: https://www.cl.cam.ac.uk/~mr10/MCPL.html but have no idea if anyone is currently using it.

[Edit: looks like Martin backported pattern matching from MCPL into BCPL ca. Oct 2022, so that's recent signs of life for both strains!]

Check "advent of code" solutions. Some of the people invent their own programming languages to solve problems.

JScript is Microsoft's JavaScript that can run server side. I guess it was a thing in the late 90s and early 00s. The "Click Commerce" web CMS/framework was largely written in it. They pivoted from commerce to medical research, and the software is still being used by dozens of institutions.

Pike (https://pike.lysator.liu.se)

Rexx at IBM, but I guess there are several such internal langs used in similar obscure companies.

Z80 assembly.

Brainfuck lol