In Mail on iCloud.com, select the message or messages you want to delete. Do any of the following: Click in the toolbar.. If you don’t see in the toolbar, click in the sidebar, then choose Preferences.In the General pane, deselect “Show Archive icon in the toolbar.” Press the Delete or Backspace key.
https://support.apple.com/guide/icloud/delete-email-mm6b1a7ab7/icloud