The problem is that passing 'transport.curl' to new Registry doesnt convert it to a nested array in the registry... whereas using ->set('transport.curl', is correctly setting it as a nested array

Code based on the SSL check when saving Joomla Global Config is like this, and works:

$options = new \Joomla\Registry\Registry;

$options->set('transport.curl',
    array(
        CURLOPT_SSL_VERIFYPEER => false,
        CURLOPT_SSL_VERIFYHOST => false,
//        CURLOPT_PROXY => null,
//        CURLOPT_PROXYUSERPWD => null,
    )
);

try {
    $response = JHttpFactory::getHttp($options, 'curl')->get('https://self-signed.badssl.com');
    echo $response->body;
} catch (\Exception $exception){
    echo '<pre>';
    var_dump($exception);
}

Clearest case:

$options = new Registry(array(
    'transport.curl'  => array(
        CURLOPT_SSL_VERIFYPEER => 0
    ),
));


$options2 = new \Joomla\Registry\Registry;
$options2->set('transport.curl',
    array(
        CURLOPT_SSL_VERIFYPEER => false,
    )
);

var_dump($options);
echo '<hr/>';
var_dump($options2);

gives:

object(Joomla\Registry\Registry)#40 (3) { ["data":protected]=> object(stdClass)#56 (1) { ["transport.curl"]=> object(stdClass)#57 (1) { ["64"]=> int(0) } } ["initialized":protected]=> bool(true) ["separator"]=> string(1) "." }
object(Joomla\Registry\Registry)#58 (3) { ["data":protected]=> object(stdClass)#59 (1) { ["transport"]=> object(stdClass)#60 (1) { ["curl"]=> array(1) { [64]=> bool(false) } } } ["initialized":protected]=> bool(false) ["separator"]=> string(1) "." }

This clearly shows the difference between using the set method and passing an array to the constructor.

My guess this is more or less expected due to how Registry accesses data. Period is by default used as a separator so using it in array keys is going to be problematic. I think this is correct code in your case:

$curlParams = array(
	'transport' => array(
		'curl' => array(
		// DO NOT DO THIS ON A LIVE SITE - NEVER EVER FOLLOW ADVICE TO DO THIS !!!!
		CURLOPT_SSL_VERIFYPEER => false,
		CURLOPT_SSL_VERIFYHOST => false,
		CURLOPT_PROXY_SSL_VERIFYPEER => false, // PHP7.3+
		CURLOPT_PROXY_SSL_VERIFYHOST => false,// PHP7.3+
		),
	),
);

I knew someone was going to try and blame "my code" when in fact all the examples are lifted straight from Joomla 3.9 code code.

Setting the registry options by constructor or by the set method should result in the same result. They don't.

You are saying that in "my case" I should not be using transport.curl - however this is exactly what Joomla core code is doing in the Gmail Authentication Plugin.

"my case" was simply debugging core code. There is no "my code"...

Registry is working fine. It's the code in Gmail plugin that's wrong. But if you disagree you can open an issue in https://github.com/joomla-framework/registry repository.

For the Gmail plugin issue see PR #30624.

If only there were unit tests ?

Registry is wrong. It’s constructor and it’s set method are acting different