As stated in #3050389: [META] Remove dependency to Classy from core themes, Classy will be moved to contrib before Drupal 9 and we have to remove dependencies on Classy from all core themes.
This meta issue will keep track of all the Umami-specific issues regarding this change.
This is process will largely consist of making Umami-specific copies of libraries and templates that were previously inherited from Classy. The specific Classy resources that Umami depends on is documented in this spreadsheet.
#3096203: Create Classy library dependency tests that can be used for all themes, and verify by providing an Umami-specific classy/dropbutton
#3106600: Decouple Classy libraries from Umami
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Comment #2
bnjmnmComment #3
bnjmnmComment #4
gábor hojtsyComment #5
bnjmnmChild issues were completed and the Classy dependency for all core themes was officially removed for Drupal 9 in #3115088: Remove Classy as a base theme of core themes. See this change record for additional details.