no because your liver makes sugars anyway. however there's a more limited, more targeted way to deplete blood of select aminoacids that some cancers can't make (asparaginase can be used this way)

it's one of these things that sound good and doesn't work. many such cases

Kinda yeah, though it is better suited as part of a combination therapy.

https://en.m.wikipedia.org/wiki/Warburg_effect_(oncology)

Yes, certain cancer therapy benefits from a zero sugar, low but high quality carb diet. You'll slow the cancer a lot, and can help prevent it from coming back like that. You'll still need something to kill it though, because your body still produces and needs sugars.

And some are unaffected because they're part of something that can already make or requires sugars, like brain or liver cancers.