PHP 8.1 introduces a deprecation for passing null to non-nullable parameters of built-in functions.
As a result, the use of explode() in commerce_checkout_paths_url_outbound_alter results in the following error:
PHP 8.1 Deprecated function: explode(): Passing null to parameter #2 ($string) of type string is deprecated
This error appears anytime that the module is enabled.
This is because template_preprocess_page() in includes/theme.inc has the line $variables['front_page'] = url();. Calling url() with no parameters leads to hook_url_outbound_alter() being passed NULL values for $path and $original_path.
Check if $original_path is a string before calling explode().
Attached is a patch that checks if $original_path is NULL and returns immediately.