Change record status: 
Introduced in branch: 
2.0.x-dev, 8.x-1.x
Description: 

The classes SettingsForm (src/Form/SettingsForm.php) and ConfigSubscriber (src/EventSubscriber/ConfigSubscriber.php) aren't supposed to be extended by other classes so they were marked as internal. In future they may be marked as final.

Impacts: 
Site builders, administrators, editors