So unless() is now just when() with 2nd and 3rd arguments swapped. We can avoid increasing the API surface by only keeping when() will 2nd and 3rd arguments both nullable (we can throw an exception if at least one of them is not passed).

so if the user wants a unless behavior they should use this?

$collection->when($user->is_admin, else: fn($it) => $it->filter(....))

I'm not sure if this is clear

Why do unless() and when() need support for else? The two methods already give you a way to do if/else style filtering logic.

when is normally a conditional 'flow' with two parts

If your condition is the result of a function you don't need to assign the value to variable,

//One method call
$collection->when(
    $Table->exists(['id' => 10]), 
    fn($it) => $it->filter(....), 
    fn($it) => $it->filter(....)
);

//instead of multiple calls
$collection->when(
    $Table->exists(['id' => 10]), 
    fn($it) => $it->filter(....),
)->unless(
    $Table->exists(['id' => 10]), 
    fn($it) => $it->filter(....),
);

//instead of using a var
$condition = $Table->exists(['id' => 10]);
$collection->when(
    $condition, 
    fn($it) => $it->filter(....),
)->unless(
    $condition,
    fn($it) => $it->filter(....),
);

To be fair I would prefer to negate the condition and have
$collection->when(!$user->is_admin, fn($it) => $it->filter(....)) than $collection->when($user->is_admin, else: fn($it) => $it->filter(....))