Tested the Patch provided in #6
Works Correctly on v9.3.0 v9.4.0
Steps Follow...

Tested the Patch provided in #6
Works Correctly on v9.3.0 v9.4.0
Steps Followed
- Install fresh Drupal setup
- Enable Carlo theme
- Create a view with pager enabled
- Added a template_preprocess_pager hook and added a test class
- Inspect the html code the preprocessed class won't be added.
- Applied the patch
- Cleared cache
- The class is added to the links.
Before Patch

After Patch

Updated the issue summary with steps to reproduce