However, if you’re like me (and you don’t want to wait) there is just one file that you need to patch. (I tried disabling the DEPRECATED warnings, however in developer mode it seems that Magento re-enables these).
The file in question is vendor/magento/framework/Encryption/Crypt.php. You need to go through the file and add the @ sign before any of the mcrypt calls (this instructs the php interpreter to suppress warnings generated from these lines).
$this->_handle = mcrypt_module_open($cipher, '', $mode, '');
$this->_handle = @mcrypt_module_open($cipher, '', $mode, '');