Is Nicomenthyl 20 a mixture of menthol and nicotinic acid or a substance?

Nicomenthyl 20 IS NOT a mixture.
It is a “substance”, per definition of substance according to art. 2 (b) of Reg. EC 1223/2009: “substance’ means a chemical element and its compounds in the natural state or obtained by any manufacturing process, including any additive necessary to preserve its stability and any impurity deriving from the process used but excluding any solvent which may be separated without affecting the stability of the substance or changing its composition”.