Updating - AbanteCart v 1.3.4 requires a min of PHP 8.0
As to updating your PHP version here is my take after 40+ years in computers, the number one reason is security, number two is performance.
Take a peek at this versions supported PHP info
https://www.php.net/supported-versions.phpSince we provide managed services as well as hosting to our clients we firmly support using current support OS and PHP versions again based on security and performance.
Unfortunately for most things in computer world as well as other areas of life - change is the only absolute. But it does keep things interesting.
Most hosting providers now have multi PHP versions. And they can be applied to different domains. So if you have one domain where you can install AbanteCart 1.3.4 - and select PHP 8.0 at the minimum that would leave your other sites running different PHP versions, Just a thought.