Can you store a generic function in a variable or pass it around without instantiating it?

No. A generic function is not a value until it is instantiated. There are no "generic function values", so f := Map fails with cannot use generic function Map without instantiation. You can instantiate it explicitly, or, since Go 1.21, let the context infer the type arguments when the target has a concrete function type.

func Map[T, U any](xs []T, f func(T) U) []U {
    out := make([]U, 0, len(xs))
    for _, x := range xs {
        out = append(out, f(x))
    }
    return out
}

// f := Map                           // compile error
f := Map[int, string]                 // explicit instantiation
var g func([]int, func(int) string) []string = Map // Go 1.21: inferred

fmt.Println(f([]int{1, 2}, strconv.Itoa)) // [1 2]
_ = g

This also explains why you cannot write a generic closure (func[T any](x T) {} is not valid syntax) and why an interface method cannot be generic. Every function value needs one concrete signature at run time. A common workaround is to capture the type parameter from an enclosing generic function, so the closure uses T but has no type parameters of its own.