Since 2017, EWWW IO has participated in an effort by a wide number of developers to encourage folks to upgrade their sites to a newer PHP release. A couple years later, the core WordPress developers joined this effort, and you can find more information about why you should upgrade here: https://wordpress.org/support/update-php/
What versions of PHP does EWWW IO support?
EWWW IO is currently supported on PHP 7.2 - 8.0. I use it on PHP 8.0 everyday, and you can see the status of the tests on various PHP versions every time I check-in the code to GitHub. It should work fine on 8.1 also, but I haven't had a chance to test it yet (we do have users who have reported it working on 8.1).