What are lesser known / taught subfields of Computer Science?

Stenography comes to mind. It seems it just gets mentioned in cryptography classes. Not sure that's very useful though.

I think machine learning is not taught enough relative to it's growing demand.

COBOL is a language that is still used in many places and will need people to replace those who are retiring, so I feel that is lesser known and not taught in many places.

Some I can think of:

- Automata theory

- Complexity theory (P vs NP stuff, not the one from leetcode problems)

- Verification

- Optimization using stuff like genetic algorithms, ant-colony algorithms, particle swarm, simulated annealing, etc. Not sure how to call that area. In my case it was an AI course.

- Fuzzy logic

- Scientific computing and Numerical analysis (CFD, finite elements, etc)

- Information theory

Program verification. How to design and implement correct programs.