Today I've been updating the Site Settings module on a Drupal project from the unsupported 8.x-1.x brach to the latest 2.0.2 version.
It's used in various custom modules on this project to inject settings needed for the module, so I needed to fix breaking changes in multiple places.
This was expected as it was a major version update and could contain breaking changes.
But this was so much easier because we'd written automated tests and used static analysis (PHPStan).