Problem/Motivation
Classy will be moved to contrib during Drupal 9. In preparation to that, we have to remove dependencies on Classy from all core themes.
Proposed resolution
Use this spreadsheet of Core Theme's use of Classy as reference
Create theme-specific versions of all libraries that are not already overridden by the theme.
Copy all remaining Templates and CSS that are not already overridden by the theme and remove dependency to Classy. Some templates in Classy are identical to Stable and can be removed instead of copied. This is covered by #3098150: Add test coverage to ensure Classy templates that have identical versions in Stable are kept in sync
Theme specific parent issues:
Claro: #3084810: Decouple Claro from Classy
Umami: #3096014: [META] Make Umami theme non-dependent on Classy
Remaining tasks
Determine directory structure for copied templates: #3095713: Create classy directory with README, in the templates and css directories for all themes subtheming Classy
Create test process for copying templates: #3096349: [PP-2] Create test for confirming Themes do not depend on Classy templates
Create test process for copying libraries: #3096203: Create Classy library dependency tests that can be used for all themes, and verify by providing an Umami-specific classy/dropbutton
Remove Classy templates with identical copies in Stable: #3098150: Add test coverage to ensure Classy templates that have identical versions in Stable are kept in sync
Once the test procedure and directory naming is settled, create multiple issues for moving templates/libraries to themes. Consider grouping similar templates in an issue to reduce the total number of issues, but the scope should remain reasonable for each.