[Plugin: Fast Secure Contact Form] Where is a $_GET['si_contact_CID'] ?

In new version si-contact-form-display.php:
Line 125:

if ( !isset($cid) && isset($_GET[$form_id_num .'mailto_id']) )
{
        $cid = (int)$this->si_contact_get_var($form_id_num,'mailto_id');
}
elseif ( !isset($cid) && isset($_GET['si_contact_CID']) )
{
	$cid = (int)$_GET['si_contact_CID'];
}

Line 149:
<!--<input type="hidden" name="si_contact_CID" value="1" />-->

The new code is improved because it is compatible with multi forms

but your complaint is valid so the next version will support legacy links.

See here for how to use the new query string feature:
https://www.fastsecurecontactform.com/query-string-parameters

The variable mailto has been changed already the third time. To have to reconsider all pages and to change it. Has persuaded me.
https://mysite/contact_page?1mailto_id=2
I suggest not to change it more.