\(\left(a-2\right)^2-\left(b-1\right)^3=1-3a-\left(-2+3b\right)\)
\(\Leftrightarrow\left(a-b-1\right)\left[\left(a-2\right)^2+\left(a-2\right)\left(b-1\right)+\left(b-1\right)^2\right]=-3\left(a-b-1\right)\)
\(\Leftrightarrow\left[{}\begin{matrix}a-b-1=0\\\left(a-2\right)^2+\left(a-2\right)\left(b-1\right)+\left(b-1\right)^2=-3\left(vn\right)\end{matrix}\right.\)
\(\Rightarrow a-b=1\)
\(\Rightarrow\left(a-b\right)^{2020}=1\)