Sorry that you had to wake up and find your site is now throwing errors. Its never fun when that happens.
Good news though, you can easily upgrade to the current version of UBB.threads, which fully supports PHP 7 and MySQLi.
Below are some versioning explanations you may find helpful:
The versions of PHP which UBB.threads 7.5.8 is compatible with, are PHP 5.2 - PHP 5.6.
The current version of UBB.threads is 7.7.1, and is compatible with PHP 5.4 - PHP 7.3.https://www.php.net/manual/en/migration70.incompatible.php
PHP 4 can connect to a MySQL database using the MySQL extension.
PHP 5 can connect to a MySQL database using either MySQL or MySQLi extension.
PHP 7 can connect to a MySQL database using the MySQLi extension (the "i" stands for improved)https://phppot.com/php/mysql-vs-mysqli-in-php/
UBB.threads 7.0.0 to UBB.threads 7.6.2 uses the MySQL extension.
UBB.threads 7.7.0 and newer uses the MySQLi extension.
When your host emails you that they are upgrading critical services, telling you to upgrade your hosted scripts to compatible versions; at the risk of any down time, its highly advised that you find out what that means, and resolve any potential issues before their date.
An example of what that email may have looked like:
[WEBHOST] is dedicated to providing you with the best resources for you and your website. So, we’re excited to announce that we will be upgrading all of our servers to newer versions of Apache and providing newer versions PHP (including PHP 7.2 & 7.3). The upgraded versions provide better data processing, which can improve the performance and speed of your website.
When will maintenance begin? We will start the maintenance process on [DATE] and will continually monitor your website. We anticipate less than 5 minutes of slowness/downtime during the upgrade itself.We recommend updating all of your web installations to the latest version--including WordPress, Joomla, Drupal or any other Content Management Tool that you may use--as soon as possible.
For custom or unique PHP code, please see the PHP Migration Guide