To facilitate easier updating of core themes, Bartik, Claro, Seven and Umami no longer depend on Stable. Prior to this change, these themes were also refactored to no longer depend on Classy. Now, all these themes set base theme: false. All templates and css not overridden in these themes will be inherited directly from core.
Extensive testing took place to determine what changes would occur if core themes inherited assets directly from core. Details on this testing can be found in these issues #3113211: Create test for theme asset decoupling from Stable and address any regressions that decoupling may cause #3110855: Plan for removing dependency to Stable in Bartik/Seven/Claro/Umami. All potentially disruptive changes were addressed within the themes themselves.
Bartik, Claro, Seven, and Umami are part of the product rather than the API and therefore it is not officially supported to extend them. These themes can change as part of a minor release and cause regressions to any themes extending them. However, if you maintain a theme that is extending Bartik, Claro, Seven, or Umami and you are using libraries extend or override to make changes to libraries in Classy or Stable, you will have to move these libraries to target another library from one of these themes or from Drupal core modules. Any templates extending Classy or Stable will also have to be modified to target either template from the theme you are extending, or from Drupal core modules.
