El teclado se accede con:

Keyboard.current

Antes de usarlo, conviene comprobar que exista:

if (Keyboard.current == null)
{
    return;
}

Tecla presionada este frame

if (Keyboard.current.spaceKey.wasPressedThisFrame)
{
    Saltar();
}

Tecla mantenida

if (Keyboard.current.wKey.isPressed)
{
    Debug.Log("W mantenida");
}

Tecla soltada

if (Keyboard.current.spaceKey.wasReleasedThisFrame)
{
    Debug.Log("Soltaste espacio");
}

Ejemplo WASD

Vector2 input = Vector2.zero;

if (Keyboard.current != null)
{
    input.x =
        (Keyboard.current.dKey.isPressed ? 1f : 0f) -
        (Keyboard.current.aKey.isPressed ? 1f : 0f);

    input.y =
        (Keyboard.current.wKey.isPressed ? 1f : 0f) -
        (Keyboard.current.sKey.isPressed ? 1f : 0f);
}

Otras teclas comunes

Keyboard.current.escapeKey
Keyboard.current.enterKey
Keyboard.current.leftShiftKey
Keyboard.current.leftCtrlKey
Keyboard.current.eKey
Keyboard.current.rKey

Resumen rápido