I would find requiring knowledge of C very odd for a web development role, unless it was a specialist backend role and you needed to interface with low level libraries.
For the vast majority of cases it shouldn't be necessary. One can even be 'full stack' in the sense that you can build and deploy a whole project without needing to be 'full stack' in the sense that you can go right to the low level coding languages in the stack.
Upshot, I don't think it's necessary.
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I would find requiring knowledge of C very odd for a web development role, unless it was a specialist backend role and you needed to interface with low level libraries.
For the vast majority of cases it shouldn't be necessary. One can even be 'full stack' in the sense that you can build and deploy a whole project without needing to be 'full stack' in the sense that you can go right to the low level coding languages in the stack.
Upshot, I don't think it's necessary.