I don't know if I'd call it obscure, but I spent a decade working in Ada.
Ada was created for high security high reliability mission critical software development, and was used extensively by governments and utilities. There was a time where simply mentioning you knew the language could land you on government watch lists.
I could tell you some of the projects I worked on with Ada, but then I'd have to kill you.
The Wikipedia page covers the basics:
I still say the dumbest thing the US government did was abandon Ada and allow C and C++ into the mix, since from a security, stability, and safety standpoint that's akin to replacing the US Army contingent at Fort Knox with two mall security rent-a-cops.
You know the old joke about shooting yourself in the foot with programming languages, where "With Pascal the compiler won't let you shoot yourself in the foot"? Well, Ada confiscates the gun, melts it down, and sends you to the nearest mental hospital for a competency assessment.
Another semi-obscure language I dealt with in the late '80's and early '90's was Eiffel.
It's an inherently object based language which, well... it's kind of like Java without the needlessly cryptic garbage from C it copied and an actually well thought out object model. It's a real shame it never caught on as working with it was really nice back in the Win 3.x days.
Even so, Eiffel Studio is still actively developed.
Back in the '80's I dealt a lot with DiBOL. It is an interesting language because frankly, it's almost identical to non line-numbered BASIC in structure and syntax, with only some different function names telling the two apart.
But because it was THE go-to for anyone running DEC mainframes for serious database use, it's still supported and used in places today where the cost -- and downtime -- of migrating software and data to newer languages and systems remains impractical. A company called "Synergex" still supports and updates the language, letting these late '70's to early '80's mainframe programs continue to lumber on running atop .NET
Let's see... how about COMAL?
Good for a laugh there was a time when the people using Commodore 64's and 128's for "business" in the mid '80's were convinced that COMAL would eventually replace all other programming languages -- something we used to laugh at since on microcomputers "no serious software could be written in anything but assembly."
Aka the days when C -- and by extension Unix -- was laughed at as a pathetic joke with needlessly pointlessly cryptic syntax, crappy compilers, piss poor memory management, and was strictly the province of "Big Iron" coders getting paid by the K-LoC. There's a reason by around 1990 C and Unix were considered technological dead-ends by anyone raised in the microcomputer "ecosystem" (I hate using that word) and only "big iron dinosaurs" cared about such. Unix and Unix-likes probably would have died off completely as a "thing" if not for Linus Torvalds.
Though I'm still not convinced this is a joke:
Oh, and you didn't list Pascal which is still alive and well no matter who says otherwise. Some major software packages are still made with Pascal via Delphi or FPC/Lazarus. Winzip, Winrar, Spybot Search and Destroy, Skype (well, the non UWP windows version at least), FL Studio, CloneDVD, Nero Burning Rom, Aida64, InnoSetup, Panda AV, IcoFX... Don't count it out of the running. When code clarity take precedence over "wah wah, I don't wanna type!", Object Pascal and/or Modula are still a favored go-to on many projects.