It’s on the type certificate that stock Lycoming and Continental engines are certified to run on 100LL. The standard for 100LL is a composition standard (not a performance standard; it literally has to have x grams of TEL/gallon, not just have a certain demonstrated octane).
Absent an STC (Supplemental Type Certificate) to change that, there is a regulation that those engines in certified aircraft have to use leaded fuel to be airworthy (legal) as airworthiness requires both “in condition for safe operation” and “in conformance with its type design”.
GAMI is pursuing an all-model (or many, many model) STC for their fuel to remedy this, but it’s a regulation issue currently.