As I commented on the #10904 issue

it looks like a later commit has undone this patch. It is not present in the current version

interface ProxyHandler<T> {
    //...
    getOwnPropertyDescriptor? (target: T, p: PropertyKey): PropertyDescriptor;   
    //...
}

The regression seems to come from commit 2e7c83abc833534181d97c534bfac865cd93b1f0 "Updated LKG"