TYPO3 Core

The current behavior caused me some trouble in v12 too: i installed typo3/cms-styleguide via composer require --dev since i only need it for my local instance to check on the registered icons.
Thus i got configuration in settings.php that was only valid for my local setup, but not the deployed instance since we skip dev dependencies.
settings.php is not writeable due to selinux and the backend module just crashed since it could not remove that section from settings.php

So if you put such config in a condition in additional.php you can't really use the backend config module properly etc., you risk accidental commits that reintroduce the config.
Having readonly config for production environments is a big plus, but currently it causes more problems than it solves