If I call collectionView.isEditing = true, nothing happens. What I'd expect is the collection/list to animate into editing mode. How is this done?
New UICollectionView list style and editing similar to UITableView?
By default, a collection view list won't necessarily look any different when editing. This is different than UITableView, where cells will automatically display certain editing accessories, depending on the methods your data source and delegate implement.
You are probably looking to have your cells display one or more editing accessories. The way you do this is by configuring the desired accessories via the accessories property on UICollectionViewListCell. For example, the insert, delete, reorder, and multiselect accessories all default to displaying only when the cell is editing.
Finally, be sure that your collection view's data source returns true from the collectionView(_:canEditItemAt:) method.