Quad Flat Package is a rectangular component package with "gull wing" leads on all 4 sides. The smallest common package is QFP-32 (8 leads on each side). The question is specifically about QFP (TQFP/LQFP/PQFP/CQFP), and not QFN.
Were QFP-28, QFP-24 or QFP-20 ever used to package electronics? I have found 3D-models of these packages, but without any reference to actual parts, just the packages. So these models could've just been made by extrapolating the specification of a higher pin-count part.
I believe, I have seen a component packaged in QFP-20. It was an RF filter, and it'sits datasheet had a technical drawing complete with all dimensions. Memory is not the most reliable source, and I couldn't find any information about it online.