You are oversimplifying the situation. I used to be a C++ backend developer and that was our stack because we needed return the response as fast as possible.

Nowadays I work as a backend developer using NodeJs and C#. I wouldn't use any of them for the former job and there are good reasons why I wouldn't use C++ for my current job. So there is such thing as right tool for the job even in one part of a web platform

There's more to computing than just websites.

Sure, but name any specific computing related thing that doesn't categorically reside within either frontend or backend. And don't say Javascript.

What is a "computing related thing"? Do transistors count? If yes, aren't they both front- and backend? What about threads? Don't they belong to both, the frontend and the backend of "computing related things"?

I'm not trying to be obtuse here, I genuinely don't understand your request.

What would you call data science/computational data stuff? Not backend, because you often sit and view the results in your R Studio console or whatever while working interactively. Not frontend because you are often directly mutating data structures and storage.

I'm working on an OCR engine right now. We intend to use it to do capturing of data using a mobile phone, but also to do verification of captured data on our backend server. So is this backend or frontend code ?

What about games? Lots of them are frontend and backend at the same time.