What it requires. Every variable declared in a flow must be referenced by at least one element, formula, or input/output assignment. Unused variables accumulate as clutter and mislead the next person about what the flow consumes and produces.
Why it matters. Dead variables make the flow lie about its own inputs and outputs, so every later change starts with working out which of them are real.
How to fix it. Delete variables that nothing references. If a variable is reserved for a planned change, record that in the flow description instead.
Source. Lightning Flow Scanner, unused-variable
Is this rule worth checking?
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