I decided it's easier to avoid using WordPress' shortcode_atts function at all.

When [example attr1="value1" attr2 attr3="value3" attr4] is parsed, the $atts parameter yields:

$atts = array (
    'attr1' => 'value1',
    0       => 'attr2',
    'attr3' => 'value3',
    1       => 'attr4'
);

You can normalize this like so:

function paragraph_shortcode ($atts, $content = null) {
    foreach ($atts as $attribute => $value) {
        if (is_int($attribute)) {
            $atts[strtolower($value)] = true;
            unset($atts[$attribute]);
        }
    }
    
    return '<p class="super-p'
        .(isset($atts['last']) ? ' last' : '').'">'
        .do_shortcode($content)
        .'</p>';
}

add_shortcode('paragraph', 'paragraph_shortcode');

When [example attr1="value1" attr2 attr3="value3" attr4] is parsed, the $atts parameter yields:

$atts = array (
    'attr1' => 'value1',
          0 => 'attr2',
    'attr3' => 'value3',
          1 => 'attr4'
);

You can normalize this like so, then you can call shortcode_atts on it.

if (!function_exists('normalize_empty_atts')) {
    function normalize_empty_atts ($atts) {
        foreach ($atts as $attribute => $value) {
            if (is_int($attribute)) {
                $atts[strtolower($value)] = true;
                unset($atts[$attribute]);
            }
        }
        return $atts;
    }
}

function paragraph_shortcode ($atts, $content = null) {
    extract(shortcode_atts(
        array (
            'last' => ''
        ),
        normalize_empty_atts($atts)
    ));
    
    return '<p class="super-p'
        .($last ? ' last' : '')
        .'">'
        .do_shortcode($content)
        .'</p>';
}

add_shortcode('paragraph', 'paragraph_shortcode');

Other notes:

• [paragraph last] would make $last = true. [paragraph last="1"] would make $last = '1'.
• Attributes are case-insensitive, so [paragraph LAST] is equivalent to [paragraph last].
• Attributes cannot be purely numeric, so the foreach loop I created is safe against integer-indexed attributes.