Ok, so this looks to have been updated only in the develop branch but not in any release, is that correct?
In 1.5.1, the Fuel_modules#add() method uses get_parent_class incorrectly (for PHP 8.0):
if (file_exists($file_path)) { …
Booting FuelCMS 1.5.1 on CodeIgniter 3.1.11 under PHP 8.0 leads to boot errors since config/fuel_modules.php tries to pass a module name as a string which doesn't seem allowed:
An uncaught Exception was encountered Type: TypeError Message: get_…