The list of Idea Workspaces had a red Delete button on every row, right beside Open. To delete one now, open it and choose More and then Delete Idea Workspace; you are asked first. More also holds Import JSON and Move to another team, which were separate buttons at the top of the workspace.
Delete an Idea Workspace from its More menu
Deleting an Idea Workspace is now in More at the top of the workspace, so the list no longer has a red Delete button beside every Open.